Diffusion furnace system for preventing gas leakage

A furnace system for preventing gas leakage is disclosed. The furnace system includes a bottom board for holding a furnace tube. A gas injection device is connected to the furnace tube at a first port for injecting reaction gas into the furnace tube. An exhaust subsystem is connected to the furnace tube at a second port. The exhaust subsystem has a first node, a second node, a third node and a fourth node. A vacuum seal is connected to the furnace tube at a third port for preventing the exhaust gas from leaking. The vacuum seal is connected to the exhaust subsystem at the first node and the second node. Finally, an N.sub.2 sealing tube is connected to furnace tube and surrounding the seam between the furnace tube and the bottom board.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to semiconductor furnaces, and more 
particularly, to an improved furnace to prevent HCl leakage. 
BACKGROUND OF THE INVENTION 
The semiconductor furnace plays an important role in the semiconductor 
manufacturing process. Common processes such as oxidation, diffusion, 
chemical vapor deposition, and annealing are performed in a furnace. A 
typical type of furnace used is the so called "lamp annealing" furnace. 
The lamp annealing furnace has the advantage that a plurality of wafers 
can be treated and the process temperature can be precisely controlled. 
However, the lamp annealing furnace does not currently provide uniform 
temperature distribution over the entire wafer surface. Another popular 
type of furnace used in the semiconductor manufacture is the "hot wall" 
furnace. There are two types of "hot wall" furnace. One is the hot wall 
horizontal diffusion furnace which is capable of controlling temperature 
over the range of 300.degree.-1200.degree. C. to an accuracy of 0.5 or 
-0.5.degree. C. over a length of up to 40 inches. 
Another type is the vertical furnace system. The major advantage of the 
vertical furnace system offers over a conventional system are (1) no 
cantilever or soft-landing is required since the wafers are held in a 
quartz boat which does not touch the process tube walls (2) the wafers can 
be loaded and unloaded automatically. (3) batch processes can be run in 
the same process tube. (4) temperature is more stable than other type. 
In VLSI fabrication, the accurate control of layer thickness and reduction 
of defects are more important than ever. Many semiconductor processes are 
performed in a furnace such as oxidation, thermal annealing, deposition, 
etc . . . . Typically, one or more kinds of gases have to be injected into 
a furnace system. For example, in a thermal oxidation process, oxygen is 
injected into the furnace so that the wafer is kept in an oxygen ambient. 
In addition, some of the processes are related to the reaction of oxygen 
and C.sub.2 H.sub.2 Cl.sub.2. It is a relatively simple reaction as shown 
below: 
##STR1## 
The gases that are generated during the process are HCl and CO.sub.2. HCl 
is a poisonous gas that is extremely harmful to humans. 
FIG. 1 shows in schematic form a conventional furnace system. A furnace 
tube 2 is used for process reaction that is set on a bottom board 4. The 
bottom board is used for setting a boat. Typically, wafers are transferred 
into the boat. A gas injection pipe 6 is connected to the furnace tube 2. 
The unreacted input gas is let through the pipe 6 into the furnace tube 2. 
An exhaust pipe 8 having a valve 10 is connected to the furnace tube 2 via 
a process gas exhaust pipe 26. The waste gas that is generated by a 
process is exhausted via the exhaust pipe 8 to an exhaust scrubber 12. 
An automatic pressure controller (APC) 14 is set midway of the exhaust pipe 
8 between the furnace tube 2 and the exhaust scrubber 12 and is used for 
maintaining and controlling the air pressure at a predetermined level 
inside the exhaust pipe 8 and the furnace tube 2. Valve 10 that is 
connected to the APC 14 is used to adjust the flow rate of the exhaust in 
the exhaust pipe 8. The APC 14 includes a sensor for monitoring the 
pressure inside the pipe 8. The mechanism of the APC 14 is controlled in 
accordance with the signal detected by the pressure sensor. For example, a 
value of air pressure is reached higher than -10 mm H.sub.2 O, the valve 
10 will be responsive to the pressure and automatically opened. When the 
pressure approaches lower than -10 mm H.sub.2 O, the valve 10 will be 
automatically closed the opening of the valve 10 a little in order to 
release exhaust out of the furnace tube 2 and keep the air pressure at the 
valve of -10 mm H.sub.2 O. 
In general, if the pressure inside the furnace tube 2 is higher than the 
pressure outside the furnace, then gas inside the furnace 2 will tend to 
leak out of the furnace tube 2 via the seam between the furnace tube 2 and 
the bottom board 4. Therefore, a vacuum seal 16 is attached to the bottom 
board 4 to prevent the gas from leaking. Typically, the vacuum seal 16 is 
connected to the exhaust pipe 8 at a node 17 via a vacuum seal pipe 20. A 
valve 10 is located midway between the node 17 and the bottom board 4 on 
the pipe 20. Another element of the furnace system is a water collector 22 
located between the node 18 and a drain assembly 24. The water collector 
22 is used to collect condensed water. The water also serves the dual 
purpose of being a pressure barrier. When water is collected by the water 
collector 22, gas cannot be exhausted from the drain assembly 24. A valve 
10 is also typically located between the water collector 22 and the drain 
assembly 24. 
In order to prevent the gas inside the furnace from leaking, the process 
gas exhaust pipe 26 has to provide a negative pressure into the furnace. 
However, the air pressure close to the process gas exhaust pipe 26 is very 
unstable due to the unstable air pressure inside the exhaust pipe 8. The 
variation of the air pressure is in the range of -10 mm H.sub.2 O to 0 mm 
H.sub.2 O. Therefore, the furnace pressure near the vacuum seal 16 is 
unstable between negative pressure and positive pressure. If the pressure 
reaches the positive pressure, then the gas will leak out of the furnace. 
Another type of prior art furnace system that is used to prevent gas 
leakage is shown in FIG. 2. The furnace system includes a furnace tube 2a 
set on a bottom board 4a. A gas injection pipe 6a is connected to the 
furnace tube 2a for injecting gas into the furnace tube 2a. An automatic 
pressure controller (APC) 14a is located midway between the furnace tube 
2a and the exhaust scrubber 12a via exhaust pipe 8a for maintaining the 
air pressure inside the furnace tube 2a. A valve 10a that is connected to 
the APC 14a to adjust the flow rate of the exhaust in the exhaust pipe 8a. 
A water collector 22a is located between the APC 14a and a drain assembly 
24a. Similarly, the water collector 22a is used to collect condensed water 
that is used as a barrier. An N.sub.2 seal 16a is connected to the bottom 
board 4a for providing positive gas pressure into the furnace tube 2a in 
order to prevent gas leakage. Typically, the N.sub.2 seal 16a is connected 
to a port 4b which is attached to the bottom board 4a. A flow meter 16b is 
connected to the N.sub.2 seal 16a for controlling the flow rate of the 
N.sub.2. 
The N.sub.2 is injected into the furnace tube 2a to generate a positive 
pressure near the bottom board 4a. This generally prevents the gas inside 
the furnace from leaking. Unfortunately, the injected N.sub.2 is forced 
into the furnace only via the port 4b. This often times allows gas to 
still leak out of the furnace tube 2a from the seam disposed away from the 
port 4b. 
Another modification of the furnace system with an N.sub.2 seal is shown in 
FIG. 3. As can be seen, an open valve 26 is connected to the exhaust pipe 
8a at node 18a. The open valve 26 will compensate for the pressure of 
process exhaust. Thus, air pressure variation will be reduced to the range 
of -9 mm H.sub.2 O to -11 mm H.sub.2 O. Although the modification can keep 
the gas exhaust pressure at the stable negative pressure. However, this 
design can not prevent the gas from leaking completely. 
SUMMARY OF THE INVENTION 
An exhaust system for a semiconductor furnace is disclosed. The furnace 
includes a bottom board and a furnace tube attached to the bottom board at 
a seam. The system comprises a main exhaust tube connected to said furnace 
tube at an exhaust port; an automatic pressure controller (APC) having an 
APC input and an APC output, said APC input connected to said main exhaust 
tube, said APC operative for maintaining the pressure inside said furnace 
tube; a liquid collector having a collector input and a collector output, 
said collector also connected to said main exhaust tube, said liquid 
collector for collecting condensed liquid for use as a barrier; a vacuum 
seal tube connected to said furnace tube at a vacuum port, said vacuum 
seal tube connecting to said APC output and also connecting to said 
collector output; and an N.sub.2 sealing means connected to said furnace 
tube and said bottom board and surrounding said seam between said furnace 
tube and said bottom board, wherein said N.sub.2 sealing means provides a 
positive pressure of N.sub.2 onto said seam at substantially the entire 
seam.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 4 shows a furnace system formed in accordance with the present 
invention. An automatic pressure controller (APC) 32 is connected to a 
process furnace tube 30 via an exhaust pipe 34. The APC is used for 
maintaining the air pressure inside the furnace tube 30. A valve 36 is 
located between the furnace tube 30 and the APC 32 to adjust the air flow 
rate of the process exhaust inside the exhaust pipe 34. A damper 38 is 
connected to the APC 32 to sever as a buffer. A second valve 40 is also 
used to adjust the air pressure of the exhaust inside the exhaust pipe 34. 
The second valve 40 is connected to the damper 38 via a node 85. Further, 
the second valve 40 is set between the APC 32 and the damper 38. A 
scrubber exhaust 42 is connected to the damper 38 to exhaust the process 
gas. An open valve 44 is attached to the exhaust pipe 34 at a node 46 to 
compensate for the pressure of process exhaust. In other words, the valve 
44 has the function of roughly adjusting the exhaust pressure and 
absorbing any disturbance from the exhaust pressure. 
A gas injection pipe 48 is connected to the furnace tube 30. The distal end 
of the gas injection pipe 48 leads to a source of gas 50 that is injected 
into the furnace tube 30 at a predetermined pressure. An N.sub.2 seal 52 
is attached to the bottom board 54. The bottom board is used to hold a 
wafer boat. Different from the prior art, the N.sub.2 seal 52 surrounds 
the entire seam that is between the furnace tube 30 and the bottom board 
54. Therefore, positive pressure is around the entire seam to prevent gas 
leakage. The distal end of the N.sub.2 seal 52 leads to a nitrogen source 
56. Between the nitrogen source 56 and the N.sub.2 seal 52 is a third 
valve 58 to control the flow rate of the nitrogen. 
A water collector 60 is connected to the exhaust pipe 34 at a node 62 to 
collect condensed water for use as a barrier. In the preferred embodiment, 
water collector 60 is formed in accordance with pending U.S. patent 
application Ser. No. 08/642,488 entitled "Furnace Exhaust System with 
Regulator" to Lin filed May 3, 1996, assigned to the assignee herein and 
expressly incorporated by reference. The node 62 is located between the 
node 46 and the APC 32. An acid drain assembly is connected to the water 
collector 60 for draining water and HCl. The assembly comprises a fourth 
valve 66, a fifth valve 68, a sixth valve 70 and a drain assembly 64. The 
fourth valve 66, fifth valve 68, and sixth valve 70 are located between 
the water collector 60 and a drain assembly 64. The fifth valve 68 and the 
sixth valve 70 serve as buffers to compensate for the pressure of process 
exhaust. 
A vacuum seal 72 is attached to the furnace tube 30 via a port 74. The 
vacuum seal 72 provides negative pressure for avoiding gas leakage from 
the furnace tube 30. The distal end of the vacuum seal 72 leads to the 
drain assembly 64 by means of a node 74. The node 74 is set between the 
sixth valve 70 and the drain assembly 64. Further, a seventh valve 76 and 
an eighth valve 78 are located between the vacuum seal 72 and the drain 
assembly 64. The function of the seventh valve 76 and the eighth valve 78 
serve as buffers to compensate for the pressure of process exhaust. 
A ninth valve 80 is connected to the vacuum seal 72 at a node 82 for 
adjusting the pressure of the vacuum seal 72. The node 82 is set between 
the vacuum seal 72 and the seventh valve 76. This arrangement is operative 
because the vacuum seal 72 will not be directly influence by the exhaust 
inside the exhaust pipe 34. The distal end of the vacuum seal 72 leads to 
the damper 38. A tenth valve 83 is located between the ninth valve 80 and 
the damper 38. A second open valve 84 is also connected to the ninth valve 
80 at node 86. Node 86 is located between the ninth valve 80 and the tenth 
valve 83. The second open valve 84 is used to compensate for the pressure 
of process exhaust and vacuum seal 72. 
The vacuum seal 72 exhausts the gas from the bottom of furnace. The N.sub.2 
seal 52 are used to provide positive pressure to the bottom board 54 to 
prevent gas leakage. The arrangement of the present invention can avoid 
unstable pressure due to the direct disruption by the exhaust pressure. 
Further, the seam that is between the furnace tube 30 and the board 54 is 
surrounded by the N.sub.2 seal 52 so that the gas will not leak from the 
furnace tube 30. In addition, the first and the second open valve 44 and 
84 are used to compensate for the pressure of the process exhaust and 
provide a more stable pressure of the exhaust. 
The scrubber exhaust 42 always keeps the vacuum condition at a pressure 
lower than -60 mm H.sub.2 O. The inner diameters of the valve 76 and valve 
78 are limited to be about 1 mm. Therefore, the gas flow rate from drain 
assembly 64 to the vacuum seal 72 will be limited to a small value. The 
APC control the pressure of process exhaust pipe 34 before the APC reaches 
the preset value of -10 mm H.sub.2 O. In addition, the output of the APC 
has been kept at a negative pressure. Thus, the vacuum seal 72 is active 
by using scrubber exhaust via the damper 38, the valve 83 and the valve 
80. 
While the preferred embodiment of the invention has been illustrated and 
described, it will be appreciated that various changes can be made therein 
without departing from the spirit and scope of the invention.