Patent Publication Number: US-6664119-B2

Title: Chemical vapor deposition method for manufacturing semiconductor devices

Description:
This application is a divisional of U.S. Pat. No. 09/183,599, filed on Oct. 29, 1998, which issued as U.S. Pat. No. 6,279,503 on Aug. 28, 2001. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a Chemical Vapor Deposition (CVD) apparatus for manufacturing semiconductor devices, its driving method, and a method of optimizing a cleaning process for the process chamber. More particularly, the invention relates to in-situ cleaning of a process chamber after processing of semiconductor wafers by using Residual Gas Analyzer Quadrupole Mass-Spectrometer (RGA-QMA). 
     DESCRIPTION OF THE RELATED ART 
     Generally, the semiconductor device fabrication process is carried out inside a process chamber having certain pre-set process conditions. In particular, when a CVD process is performed on a semiconductor wafer, a layer of material is deposited not only on the wafer, but also on the inner wall of the process chamber tube, and the boat(s) for moving the wafers between the process chamber and a loadlock chamber where the wafers are stored. As these unwanted layers are repeatedly stressed during the loading/unloading of the wafers, particles are released into the chamber that can cause defects on the wafer during the fabrication process. 
     In order to reduce the causes for defects, PM (Preventive Maintenance) is repeatedly conducted to clean the process tube at regular intervals, but the productivity of semiconductor devices fabrication is decreased due to the interruption of the process line operation. 
     FIG. 1 illustrates a conventional PM sequence for a general process tube. First, the system is cooled down after carrying out a specific process on semiconductor wafers. After the process chamber is completely cooled, the tubes of the process chamber are taken out one by one so as to carry out wet-etch cleaning of the tubes. The wet-etch generally uses chemicals such as HF group in order to remove polysilicon film or silicon nitride film from the inside of the process tube. Then, the removed tubes are assembled inside the process chamber and a vacuum test is performed. Process Recertification is carried out to see if the process chamber is ready for a new process and if the process conditions for the next process are substantially set up therein. 
     However, the above PM process represents considerable effort and expense, and takes over 24 hours to complete. Therefore, in order to overcome the problems a plasma etch of using NF 3  and CF 4  gas is carried out instead of the wet-etch. Alternatively, Thermal Shock Technology is used for removing the layers formed by thermal stress inside the chamber, or the chamber is dry-etched using ClF 3 , BrF 5 . 
     However, even though these technologies are employed, the tubes still must be removed and reassembled and the expense, the labor, and downtime remain as problems. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to provide a CVD apparatus for manufacturing semiconductor devices wherein a process chamber is equipped with a cleaning gas supply line, a sampling manifold, and a gas analyzer that are used to clean the tubes in situ. As a result, the method of the present invention substantially obviates one or more problems, disadvantages, and limitations of the prior art. 
     Another object of the present invention is to provide a method of driving the CVD apparatus, wherein a specific process is performed on wafers, and then in-situ cleaning is performed inside a process chamber, after semiconductor wafers are unloaded. 
     Another object of the present invention is to provide a method of optimizing a cleaning process for a process chamber. 
     To achieve these and other advantages and in accordance with purpose of the present invention as embodied and broadly described, a CVD apparatus of the present invention includes a process chamber in which a deposition process for manufacturing semiconductor devices is carried out; a plurality of process gas supply lines for supplying process gases to the process chamber; a waste-gas exhaust line for removing the waste-gas from the process chamber; a cleaning-gas supply line for supplying a cleaning gas to the process chamber; a sampling manifold connected to the process chamber for sampling the gas inside therein by using pressure difference; and a gas analyzer for analyzing the sampling gas from the sampling manifold. 
     Preferably, the process chamber is a Low Pressure Chemical Vapor Deposition (LPCVD) chamber having a sealed outer tube and an inner tube having an open top inside the outer tube. The cleaning gas is ClF 3 . The cleaning-gas supply line is connected to the inner tube, and the sampling manifold is connected to the outer tube. An orifice is installed in the sampling manifold such that the pressure therein is maintained at the same pressure as in the process chamber. The sampling manifold comprises a first air valve, a second air valve, a first isolation valve, a second isolation valve, a third isolation valve, and a gate valve between the connecting point with the outer tube. A purge-gas supply line is also provided in the sampling manifold. The purge-gas supply line of the sampling manifold is connected to the first air valve and the second air valve from the purge gas supply source, and third and fourth air valves are further provided between them. A Capacitance Manometer (CM) guage and a sampling pump are preferably installed between the first isolation valve and the second isolation valve of the sampling manifold in order to control the first pressure of the sampling manifold. 
     A scrubber is provided for receiving and cleaning the waste-gas passing through the waste-gas exhaust line, and the gas sampling line. 
     The gas analyzer is preferably a RGA-QMS (Residual Gas Analyzer-Quadrupole Mass Spectrometer) comprising a mass-analyzer, a turbo pump, and a baking pump, which is preferable in consideration of environmental protection. 
     The invention is also embodied in a method of driving a CVD apparatus for manufacturing semiconductor devices. The CVD apparatus includes: a process chamber; a plurality of process gas supply lines for supplying process gases into the process chamber; a waste-gas exhaust line for removing the waste-gas from the process chamber after processing; a cleaning-gas supply line for supplying a cleaning gas to the process chamber; a sampling manifold connected to the process chamber; and a gas analyzer for analyzing the sampling gas from the sampling manifold. The method comprises the steps a) sampling the gas from the process chamber; b) outgasing while baking the gas in order to reduce the initial background of the gas analyzer below a certain value; c) conducting a contamination analysis of each of the process gas supply lines; d) performing a specific process for the semiconductor wafers contained in the process chambers, e) unloading the wafers after the above specific process is completed, and exhausting the waste-gas from the process chamber; and f) cleaning the inside of the process chamber by supplying a cleaning gas thereinto. 
     The sampling manifold and the gas analyzer are continuously purged with a purge gas before conducting the sampling to ensure the precision of the gas analyzer. The contamination analysis for the process gas supply line is performed by passing nitrogen gas through each isolated process gas supply line and checking for leakage. Preferably, the fabrication process of semiconductor wafers is the one for forming a silicon-containing layer on the wafer, and the cleaning process is conducted by introducing nitrogen gas and ClF 3  gas as cleaning gases while maintaining uniform pressure inside the process chamber so that the end point of the cleaning process is easily detected. 
     The method further comprises a step of measuring particles inside the process chamber before and after the cleaning process, and the step of measuring metal/ion contaminants inside the process chamber before and after the cleaning process so as to determine the effectiveness of the cleaning process. 
     To achieve still another object of the present invention, a method of optimizing the cleaning process for a process chamber, the cleaning process carried out in-situ after a specific process is performed for a wafer placed inside the process chamber, with a cleaning gas supply line for sup plying the cleaning gas into the process chamber, a sampling manifold connected to the process chamber, and a gas analyzer for analyzing the sampling gas from the sampling manifold. The method comprises: a) after performing a specific process on the semiconductor wafer, cleaning the process chamber by supplying a certain amount of nitrogen gas and ClF 3  as cleaning gas while maintaining a constant pressure and temperature inside the process chamber until the cleaning end point is detected by the gas analyzer; and b) after performing the same specific process for another semiconductor wafer, cleaning the process chamber by supplying a certain amount of nitrogen gas and ClF 3  as cleaning gas, and varying the pressure and the temperature inside the processing chamber until the cleaning end is detected point by the gas analyzer. 
     Preferably, the end point of the gas analyzer is determined by the intersecting point of amplified traces for an etch gas and the etching byproducts. 
     According to the present invention, when a specific process is performed on a semiconductor wafer and the cleaning process is carried out inside the process chamber by using ClF 3  gas, the mechanism is exactly monitored, and the composition of the cleaning process is optimized to simplify the process and to improve process efficiency. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a brief representation showing the conventional sequence of a cleaning process for the process tube of the conventional Chemical Vapor Deposition (CVD) apparatus for manufacturing semiconductor devices; 
     FIG. 2 is a schematic representation showing the CVD apparatus for manufacturing semiconductor devices according to one embodiment of the present invention; 
     FIG. 3 shows a sequence for the process analysis and the cleaning process in the CVD apparatus of FIG. 2 according to one embodiment of the present invention; 
     FIG. 4 shows an analysis trend for the storage polysilicon deposition process according to one embodiment of the present invention; 
     FIG. 5 shows an analysis trend for the cleaning process according to one embodiment of the present invention; 
     FIG. 6 is a graph correlating etch rate in the cleaning process to pressure inside a process chamber according to one embodiment of the present invention; 
     FIG. 7 is a graph correlating etch rate in the cleaning process to temperature inside a process chamber according to one embodiment of the present invention; and 
     FIG. 8 is a graph correlating etch rate in the cleaning process to ClF 3  flow inside a process chamber according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 2 is a schematic representation showing the CVD apparatus for manufacturing semiconductor devices according to one embodiment of the present invention. A process chamber  10  comprises an outer tube  14  and an inner tube  16 . Inside the process chamber  10 , various processes such as deposition process, plasma process, diffusion process, or CVD process, etc. are performed. A loadlock chamber  12  is installed below the process chamber. A boat  18  for holding wafers to be processed is moved up and down between the process chamber  10  and the loadlock chamber  12  by an elevator  20 . A gas supply line  22  for supplying process gas for processing is connected to the lower side of the inner tube  16 . The gas supply line  22  may have a separate pipe line and valve for each process gas or cleaning gas. In the system shown in FIG. 2, individual process gas supply lines and valves  32 ,  34 ,  36 ,  38 ,  40  are provided for SiH 4  supply source  24 , PH 3  supply source  26 , N 2  supply source  28 , and ClF 3  supply source  30  respectively. The ClF 3  supply source  30  is a cleaning gas supply source, which will be mentioned below. The gas supply line  22  may have a separate pipe line for each process gas or cleaning gas. 
     Meanwhile, the process waste-gas is evacuated from outer tube  14 , through a discharge line  42 , by a discharge pump  44 , and is then routed to scrubber  46  for cleaning. 
     In order to monitor the gas composition inside the process chamber  10 , a sampling port  48  is installed in the outer tube  14 , The sampling port  48  is connected to a sampling manifold  50 , preferably by using a flexible connecting line  52 . Sampling line  54  of sampling manifold  50  is made of ⅓ inch diameter electropolished stainless steel pipe. Flow-through sampling line  54  is controlled by a first air valve  62 , a second air valve  66 , a first isolation valve  68 , a second isolation valve  70 , a third isolation valve  72 , and a gate valve  74 . The first isolation valve  68  and the second isolation valve  70  are each fitted with a 100 micron orifice; and the third isolation valve  72  is fitted with a 250 micron orifice. 
     The sampling manifold  50  has an N 2  supply source  56  for use as a purge gas, which is available whether or not samples are being drawn. The vacuum system can be damaged by the concentration of the gas due to a small amount of water inside the Gas Distribution System (GDS) so that it is very important to precisely control the purge cycle or cleaning time for the process chamber. The N 2    58  is connected to the first air valve  62  and to the second air valve  66 . Further, a CM gauge  76  is installed between the first isolation valve  68  and the second isolation valve  70  on sampling line  54 . Sample line line  54  is connected to sampling pump  90 , which discharges to the scrubber  46 . 
     Meanwhile, sampling line  54  is connected to a gas analyzer  80  through gate valve  74 . The gas analyzer  80  uses a commercially available RGA-QMS (Residual Gas Analyzer-Quadrupole Mass Spectrometer)  84 . An ion gauge  82  is installed on the mass analyzer  84 . Sample gases pass through a turbo pump  86  and a baking pump  88 , and to the scrubber  46 . 
     Meanwhile, the ClF 3  gas for use in the present invention is a cleaning gas that can be also used in the cleaning of polysilicon, silicon nitride, silicon glass, and tungsten silicide. It can be used in the low temperature state as well as a plasma state, and has excellent chemical selectivity so that it performs etching at the portions where plasma cannot reach. It also has the advantage that it is highly unlikely to generate particles that could contaminate the wafer surface. In use ClF 3  is generally diluted to a concentration of 20±5 volume % with an inert gas such as N 2 . While the lower pressure in the process chamber is good for the uniform etch for the layer inside the chamber, the higher mixing rate of etch gas is good for increasing the etch rate. It is preferable to heat the process chamber to a temperature higher than the boiling point of the ClF 3 , prior to the introduction of ClF 3  and preferably higher than 400° C. for the desirable etch rate. Since ClF 3  is a very reactive gas, if the etch rate is too high, tubes  14  and  16  could also be etched, shortening the useful life of the tube. 
     The ClF 3  supply pipe is preferably formed of nickel, monel hastelloy, 316L stainless steel, or a polymeric material due to the properties of the ClF 3 . 
     Meanwhile, the RGA-QMS (Residual Gas Analyzer-Quadrupole Mass Spectrometer) used as the gas analyzer  80  is operated in such a manner that sampled gas from the process chamber is ionized by bombardment with electrons accelerated with 70 eV. Then, Quadrupole Mass Spectrometer passes only those ions having a specific rate of mass to electric charges so as to obtain a mass spectrum. By the composition of the ions achieved from the above results, the composition of the sampled gas can be determined. The RGA-QMS used in the present invention is a portable system, and unlike general OIS (Open Ion Source) used in the sputtering process, the ion source is a CIS (Closed Ion Source) so that it is possible to analyze, the process gas as well as bulk gas. 
     The sampling pressure is controlled uniformly below the process chamber pressure by the orifices (100/250 micron) inside the sampling manifold  50 . 
     Turning now to FIG. 3, a sequence for the process analysis and the cleaning process in the CVD apparatus of FIG. 2 will now be described. First, gas analyzer  80  is connected to the sampling manifold. N 2  gas is continuously supplied to purge the RGA-QMS. First air valve  62  and third air valve  60  are closed; second air valve  66  and fourth air valve  64  are opened. Then, fourth air valve  64  is closed, and first air valve  62  is open, and the gas inside the process chamber  10  is sampled. At this stage, if it is necessary to control the pressure in both the process chamber  10  and the sampling line  54 , which can be done by operating a sampling pump  90  based on the pressure indicated on a CM gauge  76 . 
     Then, the RGA-QMS baking evaluation is conducted. That is, after placing a quadrupole mass spectrometer inside an RGA-QMS chamber (not shown), baking is carried out in order to decrease the background. Since RGA-QMS is very sensitive to contamination, its contamination level is determined by analyzing the background spectrum as part of every test to measure contamination if any, by water and oxygen. When the contamination level is to high, the RGA-QMS chamber is baked at about 250° C., and the sampling manifold is baked at about 150° C. so as to reduce the contamination level. During baking, the partial pressure of each molecular contaminant (H 2 O, H 2 , O 2 , Ar, CO 2 , etc.) is monitored. The outgasing of the contaminants is accelerated through the baking so that the background of the RGA-QMS is reduced. 
     Sequentially, the contamination for the gas line is analyzed, -in order to analyze the integrity of each supply line (SiH 4 , PH 3 , and N2). N 2  gas at 500 SCCM is introduced into each supply line, one by one. Gas inside the process chamber is then sampled and analyzed to determine if the gas supply line is leaking. 
     Then, a specific process for semiconductor wafers is carried out, and sampling is carried out so as to analyze the process. At this time, for example, in a storage-polysilicon deposition step of DRAM processing, continuous sampling may be carried out in the prepurge and after-purge step as well as in the deposition step. The manifold is maintained below the 0.9 Torr pressure of the process chamber, by way of the critical orifice on the sampling manifold of the RGA-QMS. FIG. 4 shows a typical analysis trend for the storage-polysilicon formation process. 
     After the specific process for semiconductor wafers is completed, the boat containing the wafers is unloaded from a process chamber, and the waste-gas therein is discharged. Then, a ClF 3  in-situ cleaning process is carried out. In the case of depositing storage-poly layer to a thickness of 48,000 Å, the adhered layer inside the process chamber is etched away using a mixture of 2800 SCCM N 2  gas and 700 SCCM ClF 3  gas. 
     Then, cleaning process steps are analyzed to determine the EPD (End Point Detection) of the cleaning process. The cleaning process is analyzed by alternately varying the pressure and temperature of the process chamber while maintaining the flow of cleaning gas (for example, N 2  gas of 2800 SCCM, and ClF 3  gas of 700 SCCM). Like the storage-poly formation process analysis, the EPD analysis of the cleaning process is performed throughout the whole cleaning process. FIG. 5 shows an analysis trend for the in-situ cleaning process by ClF 3  after storage-poly deposition process. As shown in FIG. 5, the cleaning process is divided into three steps. The first step is evacuating and purging the chamber prior to cleaning, as shown in FIG. 5, during the SCCM time from 0 to 50. The ClF 3  etching is carried out in the second step, and is represented in FIG. 5 from 50 to about 280 scan. The third step is the evacuation and purging after the etching is complete, which corresponds to the time after 280 scan. 
     As shown in FIG. 5, the EPD of the cleaning process is the point indicated by a bold down arrow around 280 scan. At that point (scan=280), the concentration of HF+ equals the concentration of SiF3+ in the process chamber. At that point, the ClF 3 -N 2  gas mixture has largely etched away the silicon-containing layers on the inside walls of process chamber  10 . After that point, continued flow of etchant gas into the process chamber results in unwanted etching of the polysilicon layer by the fluoride and chloride radicals generated by ClF 3 . The unwanted etching of the polysilicon layer is indicated, according to the invention, by the detection of HF. HF is a polysilicon etching byproduct that is not present in relatively high concentrations during the etching of silicon from the process chamber walls. By repeatedly carrying out the same storage-poly deposition process by varying the pressure and temperature of process chamber  10 , the flow rate of the ClF 3 , and determining EPD for each case, the process time for each step of the cleaning process can be optimized. The results of the optimization as above are shown in FIGS. 6,  7  and  8 . FIG. 6 is a graph correlating etch rate in the cleaning process to the pressure inside a process chamber  10 . FIG. 7 is a graph correlating the etch rate during the cleaning process to the temperature inside process chamber  10 . FIG. 8 is a graph correlating etch rate in the cleaning process to ClF 3  flow rate inside a process chamber. The effectiveness of the cleaning can be evaluated by monitoring the particles present in process chamber  10  before and after the cleaning process. Metal and ion contaminants such as Fe, Cr, Ni, Zn, Ti, S, Cl, F, NH 4  can also be monitored using TXRF/HPIC (Total X-ray Reflection Fluorescence/High Performance Ion Chromatography) before and after the cleaning process. 
     Therefore, according to the present invention, without removing the process tubes, the process chamber can be in-situ cleaned so that the life of process chamber is increased, the cleaning time is shortened, and productivity is improved. 
     In addition, according to the present invention, the in-situ cleaning process can be optimized so that the life of the process chamber is increased and cleaning time is shortened. Further, according to the present invention, the processes for the wafers continuously is monitored, and analyzed so that the process malfunctioning is prevented contributing to the increase in productivity. Still further, while the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made hereto without departing from the spirit and scope of the invention as defined by the appended claims.