1. Field of the Invention
The present invention relates to a film deposition apparatus by a plasma enhanced CVD process.
2. Description of the Related Art
A plasma enhanced CVD process has been known as a method for forming a deposited film on a substrate. In the plasma enhanced CVD process, a gas supplier, a gas exhauster, and a plasma generator are provided in a vacuum chamber, and a deposited film is formed on a substrate placed into the vacuum chamber by means of plasma generated from material gases. For example, material gases, such as SiH.sub.4 and Si.sub.2 H.sub.6 are supplied into the vacuum chamber, and an RF- or microwave-discharging electrical power is applied to generate plasma in the vacuum chamber while evacuating to keep a constant pressure. An amorphous silicon semiconductor film is thereby formed on the substrate surface in the vacuum chamber.
Conventional plasma apparatuses produce particles or powder as by-products in the formation step of the deposited film, these particles deposit in the vacuum chamber, and in particular, near the vent of the exhauster. Adhesion of such particles on the substrate surface deteriorates the characteristics of the deposited film and causes formation of defects such as pinholes, resulting in a decreased yield of products using the deposited films. For example, an amorphous silicon solar battery produced by a plasma enhanced CVD process requires a semiconductor device with a significantly large area; however, if particles are present in the amorphous silicon deposited film, the resulting solar battery device will have defects such as short-circuiting, because the amorphous silicon film has a significantly low thickness of less than approximately 500 nm and a layered structure of pin, p-i-n, p-i-n-p-i-n or p-i-n-p-i-n-p-i-n.
U.S. Pat. No. 4,400,409 discloses a continuous plasma enhanced CVD apparatus using a roll-to-roll system for continuously producing amorphous silicon semiconductor devices. The CVD apparatus is provided with a plurality of plasma enhanced CVD chambers that are arranged along a path so that a strip of long substrate with a desired width continuously travels in these plasma enhanced CVD chambers. Required conductive-type semiconductor films are separately deposited in these plasma enhanced CVD chambers while transferring the substrates along the path. Devices that have semiconductor junctions and a large area are thereby continuously formed. Consequently, the roll-to-roll-type continuous plasma enhanced CVD apparatus can continuously produce devices for a long time with high productivity.
On the other hand, such continuous film deposition causes ready accumulation of particles in the plasma enhanced CVD chambers under certain conditions. Since accumulated particles increase with the production time, the resulting devices will have defects. Further, evacuating systems for evacuating the plasma enhanced CVD chambers will malfunction because of clogging by the particles, resulting in a decreased operation rate.
Several methods for suppressing adhesion of the particles are disclosed in, for example, Japanese Patent Laid-Open Nos. 60-114570, 1-312833, 4-136175, 8-133899, and 8-299784.
In the method disclosed in Japanese Patent Laid-Open No. 60-114570, exhaust pipes and trapped particles or powder are heated so that the trapped powder is converted into high-density, hard, fine powder which is easily collected.
In the method disclosed in Japanese Patent Laid-Open No. 1-312833, a second reaction chamber and a second heating means are provided between the hermetic chambers and the evacuation systems so as to completely decompose gas which is not pyrolyzed in the first reaction chamber.
The thin film deposition apparatus disclosed in Japanese Patent Laid-Open No. 4-136175 is provided with reaction chambers for depositing films by the reaction of the unreacted gas in the exhaust gas so as to decrease untreated gas in the exhaust pipes, in order to prevent the formation of dust, such as powder or film fragments, and to prevent deterioration of vacuum pumps and toxic gas treating units.
In the technology disclosed in Japanese Patent Laid-Open No. 8-133899, films are deposited while heating exhaust pipes that pass through the interiors of substrate heaters.
In the technology disclosed in Japanese Patent Laid-Open No. 8-299784, a heating trap composed of a baffle-plate is provided in a passage container which intervenes in the midway of each evacuation path in order to decompose and substantially trap the unreacted gas in the exhaust gas.
These technologies are capable of effectively collecting or disposing unreacted gas in the exhaust pipes, powder formed by the reaction of the unreacted gas, and film fragments, and of solving the considerable problems caused by the unreacted gas, powder, and film fragments.
These conventional plasma enhanced CVD technologies have been developed to solve problems occurring in the exhaust pipes, and thus do not suggest problems occurring at the interior and/or vicinity of the plasma discharging space. That is, the conventional technologies suppress problems in the evacuation systems, such as decreased evacuation efficiency and deteriorated pump oil, caused by unreacted gas in the exhaust pipes, and contamination of substrates and deposited films caused by backflow of powder formed in the exhaust pipes into reaction chambers. Thus, in the continuous film deposition by a roll-to-roll process, problems caused by particles or powder deposited on the plasma enhanced CVD chambers still remain.
Particles or powder may deposit on places other than the exhaust pipes, for example, inner walls of the reaction chambers which form plasma discharging spaces, and particularly near the discharging spaces and the exhaust pipes. The deposited particles will spontaneously drop or be scattered by a gas stream or a backflow from the evacuation system during a lengthy film deposition operation in the plasma enhanced CVD process. These particles will deposit on the substrates or the top surface of the film under the deposition process, and may cause defects in the deposited film. The use of a gas gate causes a scavenging gas flow in addition to the material gas flow.
The above-mentioned conventional technologies cannot substantially solve such problems caused by particles depositing on the places other than the exhaust pipes.
In the conventional technologies, exhaust pipes generally have simplified inner shapes, such as cylinders or prisms, to suppress the contact area with exhaust gas, and are heated to prevent adhesion of particles.
Although the technologies can prevent adhesion of particles on the interiors of the exhaust pipes during long, continuous film deposition operations, these cannot prevent adhesion of particles on unheated places, such as pipe walls and vacuum pumps, and portions having complicated shapes, such as exhaust valves. The adhered particles cause malfunction of the valves and other units, and the particles are inevitably scattered into the reaction chambers.
In some technologies, powder traps are provided midway along the exhaust pipes to collect particles or powder in order to prevent adhesion of particles on the unheated places, such as inner walls of the exhaust pipes and vacuum pumps, and on places having complicated shapes, such as exhaust valves; however, the traps will easily clog and clogged powder is difficult to remove. Thus, the evacuation efficiency decreases and life of the pumps is shortened.