Exhaust system for chemical vapor deposition apparatus

An exhaust system comprising an exhaust device and a collecting device incorporating a filter and positioned in front of the exhaust device, the exhaust system being adapted for use in a chemical vapor deposition apparatus, wherein at least a part of the collecting device is heated.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to collecting means for powder by-products 
generated in a decomposition process of a raw material gas in a chemical 
vapor deposition apparatus. 
2. Description of the Prior Art 
The chemical vapor deposition (CVD) process has recently been attracting 
attention as a forming method for the insulating layer for integrated 
circuits, solar cells and electrophotographic photosensitive members, in 
view of the usefulness of the process at a relatively low temperature, and 
of the superiority of the properties of the obtained layer, and has 
already been partly employed in commercial production in those fields. 
The CVD process is generally performed by introducing gaseous raw material 
molecules into a deposition chamber in which pressure can be reduced by an 
exhaust system and decomposing the raw material gas with an excitation 
energy to deposit a desired layer on a desired substrate. In such process, 
the function of said exhaust system is often hindered by by-products of 
powder form (hereinafter called fine powder) which are generated in the 
course of deposition of the raw material gas. As an example, in the 
formation of a layer of silicon hydride-nitride, silicon 
hydride-nitride-oxide, silicon hydride-oxide, silicon hydride or 
silicon-germanium hydride, fine powder containing silicon is generated as 
a by-product and deposited in the exhaust system, significantly degrading 
the exhaust ability. It therefore becomes not only difficult to maintain a 
constant gas pressure in the deposition chamber over a prolonged period 
but also difficult to obtain a layer having desired properties since the 
gas pressure in the deposition chamber gradually rises even in a single 
operation whereby the raw material gas concentration and the discharge 
between the electrodes become unstable. 
The conventional CVD apparatus is generally equipped with a filter in front 
of the exhaust system in order to eliminate such fine powder generated in 
the layer deposition, but said fine powder, being soft at a low 
temperature and easily clogging such filter, not only deteriorates the 
productivity significantly but also undesirably affects the properties of 
the deposited layer. 
SUMMARY OF THE INVENTION 
In consideration of the foregoing, the object of the present invention is 
to provide a process for collecting the fine powder generated in the 
formation of a deposition layer through the CVD process, said process 
being free from the drawbacks of the abovedescribed conventional processes 
for fine powder collection. 
The aforementioned object can be achieved according to the present 
invention, in an exhaust system for a CVD apparatus composed of an exhaust 
device and a collecting device equipped with a filter and positioned in 
front of said exhaust device, by heating said collecting device, or 
heating said collecting device and exhaust pipings to said collecting 
device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention will now be clarified in detail by an embodiment 
shown in the attached drawing. FIG. 1 schematically shows a plasma CVD 
apparatus, as an example of the CVD apparatus, provided with an exhaust 
system according to the present invention. In FIG. 1, there are shown a 
deposition chamber 101 for forming a deposited layer, which is connected 
to an exhaust system through a main valve 113; a raw material gas inlet 
102 for introducing the raw material gas into the deposition chamber; a 
substrate 106 supported by a support member 107 and functioning as a 
grounded anode; and a discharge power source 103 which applies, through a 
matching box 104, a high voltage of a desired frequency, usually 13.56 
MHz, to an electrode 105, thereby decomposing the raw material gas and 
depositing a layer onto the substrate. The composition or structure of the 
raw material gas, deposition chamber, substrate, electrodes, etc., is not 
critical and may be arbitrarily selected for producing the desired 
deposited layer. 
There are also shown an oil rotary pump 108 and a mechanical booster pump 
109 constituting an exhaust device for reducing the pressure in the 
deposition chamber, generally to a vacuum of 0.5 to 1.0.times.10.sup.-3 
Torr; a fine powder collecting device 110 provided with a filter 112 and 
positioned in front of said exhaust device for preventing smearing of the 
exhaust device by the fine powder; and a heater 111 connected to an 
unrepresented heating power source and positioned to heat said collecting 
device and an exhaust piping between said collecting device and the main 
valve (hereinafter called front exhaust piping). The heating is conducted 
to a temperature to sufficiently harden the fine powder to avoid the 
filter clogging, namely 100.degree. C. or higher in the collecting device 
and in the exhaust front piping. After said exhaust device there is 
generally provided, though unrepresented, a scrubber for eliminating toxic 
or corrosive gas employed in the reaction by means of an aqueous solution 
or a catalyst such as Toxoclean prior to the discharge into the air, and a 
nitrogen gas introducing line for diluting combustible gas such as silane 
to a concentration below the explosion limit. 
The present invention is based on the fact that the fine powder generated 
in the layer deposition in the CVD process becomes harder at higher 
temperatures, and is therefore characterized by heating the collecting 
device and the exhaust front piping, and such heating facilitates the 
collection of the fine powder and resolves the troubles in the exhaust 
system such as the lowered exhaust ability of the exhaust device, thus 
enabling one to stably obtain a deposited layer of desired properties and 
to improve the productivity. 
According to the present invention it is desirable to heat both the 
collecting device and the exhaust front piping, but it is also possible to 
achieve the same effects depending on the species or flow rate of the raw 
material gas, state of generation of the fine powder and the applied 
voltage. 
The heating method and the heat source may be arbitrarily selected as long 
as the collecting device and the exhaust front piping can be sufficiently 
heated, and, for example, there may be employed a method of passing steam 
or hot air into a pipe provided in said devices, or a method of electrical 
heating by heaters such as nickel-chromium wires provided on said devices. 
Though the heating method is not limited, it is preferable to heat said 
devices from the outside rather than using internal heating, since 
combustible gases are often used in said devices. 
In the present invention there may be employed any filter that is heat 
resistant and is capable of satisfactorily collecting granular substances 
contained in the exhaust gas from the reaction chamber, but a filter 
incorporating a mesh for example of stainless steel is preferable in 
consideration of the ease of maintenance. The mesh size is determined to 
satisfactorily collect the granular substance without affecting the 
exhaust capacity of the vacuum pump, and is preferably in a range from 100 
to 10,000 mesh. Plural filters of the same or different mesh sizes may be 
employed depending on the status of generation of the fine powder. Also 
said filter is preferably constructed as a cassette filter for 
facilitating periodical discharge of the collected powder. 
The exhaust device to be employed in the present invention can be of any 
type that can maintain a vacuum of 0.5 to 1.0.times.10.sup.-3 Torr in the 
deposition chamber, and there may generally be employed an oil rotary 
pump, a mechanical booster pump, a turbo molecular pump, etc. Among these 
pumps is preferred a combination, from the side of the deposition chamber, 
of a mechanical booster pump and an oil rotary pump, of a turbo molecular 
pump and an oil rotary pump or of a turbo molecular pump and a sorption 
pump (for example a molecular sorption pump), and particularly preferred 
is the combination of a mechanical booster pump and an oil rotary pump. 
The present invention will be understood further from an example. 
EXAMPLE 
In the plasma CVD apparatus shown in FIG. 1, the collecting device and the 
exhaust front piping were provided with ribbon-shaped heaters to maintain 
said collecting device and said piping at 250.degree. C. There were 
employed two filters incorporating stainless meshes of 200 mesh size. 
SiH.sub.4 gas diluted with H.sub.2 gas to a concentration of 10 vol.% was 
employed as the raw material gas and was supplied at a flow rate of 100 
SCCM at a pressure of 0.3 Torr in the deposition chamber. A high frequency 
voltage of 13.56 MHz was applied across the electrodes to form an 
amorphous silicon layer on an aluminum substrate. 
In the operation without heating, the pressure in the deposition chamber 
showed gradual increase in three batches, and troubles such as filter 
clogging were observed in the following batches. On the other hand, in the 
operation with heating, the pressure in the deposition chamber was stable 
even after twenty batches and the deposited layer with desired properties 
could be obtained in stable manner without the troubles such as filter 
clogging.