Apparatus for vacuum line cleaning in substrate processing equipment

Abstract: An apparatus for minimizing deposition in an exhaust line of a substrate processing chamber. The apparatus (40) includes first and second electrodes (56,58) having opposing surfaces that define a fluid conduit (54) between them. The fluid conduit includes an inlet (50), an outlet (52) and a collection chamber between the inlet and the outlet. The apparatus is connected at its inlet (50) to receive the exhaust of a substrate processing chamber. The collection chamber is structured and arranged to collect particulate matter flowing through the fluid conduit and to inhibit egress of the particulate matter from the collection chamber. A plasma generation system (102) supplies power to the electrodes to form a plasma from etchant gases within the fluid conduit. Constituents from the plasma react with the particulate matter collected in the collection chamber to form gaseous products that may be pumped out of the fluid conduit. The apparatus may further include an electrostatic collector (100) to enhance particle collection in the collection chamber and to further inhibit egress of the particulate matter.

Description: The present invention relates to a method and apparatus for minimizing deposition in an exhaust line of a substrate processing chamber, and in particular to a method and apparatus for eliminating contaminants and residues from inside a vacuum exhaust line connected to a processing chamber and/or to a method and apparatus for reducing perfluorocompound (PFC) gas emissions from a processing chamber. During chemical vapor deposition (CVD) processing, deposition gases are released inside a processing chamber to form a thin film layer on the surface of a substrate being processed. Unwanted deposition on areas such as the walls of the processing chamber also occurs during such CVD processes. Because the residence time in the chamber of individual molecules in these deposition gases is relatively short, however, only a small portion of the molecules released into the chamber are consumed in the deposition process and deposited on either the wafer or chamber walls. The unconsumed gas molecules are pumped out of the chamber along with partially reacted compounds and reaction byproducts through a vacuum line that is commonly referred to as the "foreline." Many of the compounds in this exhausted gas are still in highly reactive states and/or contain residues or particulate matter that can form unwanted deposits in the foreline. Given time, this deposition build-up of powdery residue and/or particulate matter presents a problem. First, the matter is often a pyrophoric substance that may present problems when the vacuum seal is broken and the foreline is exposed to ambient conditions during standard, periodic cleaning operations. Second, if enough of the deposition material builds-up in the foreline, the foreline and/or its associated vacuum pump may clog if it is not appropriately cleaned. Even when periodically cleaned, matter build-up interferes with normal operation of the vacuum pump and can drastically shorten the useful life of the pump. Also, the solid matter may backwash from the foreline into the processing chamber and contaminate processing steps adversely affecting wafer yield. To avoid these problems, the inside surface of the foreline is regularly cleaned to remove the deposited material. This procedure is performed during a standard chamber clean operation that is employed to remove unwanted deposition material from the chamber walls and similar areas of the processing chamber. Common chamber cleaning techniques include the use of an etching gas, such as fluorine, to remove the deposited material from the chamber walls and other areas. The etching gas is introduced into the chamber and a plasma is formed so that the etching gas reacts with and removes the deposited material from the chamber walls. Such cleaning procedures are commonly performed between deposition steps for every wafer or every N wafers. Removal of deposition material from chamber walls is relatively straightforward in that the plasma is created within the chamber in an area proximate to the deposited material. Removal of deposition material from the foreline is more difficult because the foreline is downstream from the processing chamber. In a fixed time period, most points within the processing chamber come in contact with more of the etchant fluorine atoms than do points within the foreline. Thus, in a fixed time period, the chamber may be adequately cleaned by the clean process while residue and similar deposits remain in the foreline. To attempt to adequately clean the foreline, the duration of the clean operation must be increased. Increasing the length of the clean operation, however, is undesirable because it adversely affects wafer throughput. Also, such residue build-up can be cleaned only to the extent that reactants from the clean step are exhausted into the foreline in a state that they may react with the residue in the foreline. In some systems and applications, the lifetime of the exhausted reactants is not sufficient to reach the end or even middle portions of the foreline. In these systems and applications, residue build-up is even more of a concern. Accordingly, there is a need for an apparatus for efficiently and thoroughly cleaning the foreline in a semiconductor processing system and a method of doing the same. From the EP 0 289 858, a fine particle collector trap for vacuum evacuation systems is known therein a vessel formed of a double-wall cylinder and having an inlet conduit connected to a vacuum processing chamber and an outlet conduit connected to at least one vacuum pump is provided. A passage for flowing gas is defined between the double walls of the vessel, and one of the double walls is heated and the other wall is cooled so as to maintain a predetermined temperature differences therebetween. Furthermore, a plasma may be struck between the cover of the cover of the vessel and the inside of the outer cylinder. Both the activation of cleaning gas and the chemical reaction of the particles adhered to the low temperature wall are performed in the vessel. From JP-A-1-288355 the recovery of particles from the gas produced in a CVD reactor is known, therein a discharge is formed between discharge and collector electrodes to charge the particles, and the particles are, thereafter, recovered electrostatically on a collector surface disposed near a collector electrode. From US-A-5,211,729 a baffle/settling chamber is known which removes solid particles from the exhaust of a semiconductor deposition equipment. A container having an inlet baffle plate, an outlet baffle plate and three settling plates disposed therebetween are disposed in a chamber and reside serially in the exhaust flow. A fluid passes between the opposing surfaces of the baffle plates, and a significant amount of particles drops to the bottom of the container forming a mechanical trap. It is the object of the invention to provide an effective method and apparatus for eliminating contaminants and residues from inside a vacuum exhaust line connected to a processing chamber. The present invention solves the above problems of the prior art by providing a method and an apparatus as claimed in the independant claims, that substantially prevent particulate matter and other residual material from building up in an exhaust line of a substrate processing chamber. The present invention achieves these goals while being process transparent. That is, in preferred embodiments, operation of the present invention takes no additional processing time to either prevent particulate matter from building up within the foreline or reduce PFC emissions as appropriate. Also, in some preferred embodiments, the present invention does not require the use of additional gases and/or consumable parts. In one version, the apparatus of the present invention includes first and second electrodes having opposing surfaces that define a fluid conduit. The fluid conduit has an inlet, an outlet and a collection chamber between the inlet and the outlet that is structured and arranged to collect particulate matter flowing through the fluid conduit and to inhibit egress of the particulate matter from the collection chamber. A microwave plasma generation system is operatively coupled to the apparatus to form a plasma from etchant gases within said fluid conduit. Constituents from said plasma react with the particulate matter collected in the collection chamber to form gaseous products that may be pumped out of the fluid conduit. In preferred versions of this embodiment of the apparatus, the first and second members are each electrodes and the apparatus also includes a particle trapping system that applies a voltage between the two electrodes to collect particulate matter on the electrode surfaces. The plasma also reacts with this electrically collected matter to convert the matter to gaseous products that may be pumped out of the apparatus. The gas passageway includes at least one collection chamber that is structured and arranged such that gravitational forces act to collect particulate matter flowing through the passageway and inhibit egress of the particulate matter from the collection chamber. Additionally, a voltage is applied to at least one of the electrodes to create a voltage field between the electrodes that helps collect and trap particulate matter flowing through the passageway. In another version, the apparatus of the present invention includes a pair of capacitively coupled electrodes define a labyrinthal gas passageway. DC or AC voltage is applied to the electrodes to create a voltage field within the passageway. The voltage field attracts negatively charged particles exhausted through the passageway on one electrode and positively charged particles on the other electrode. The defined passageway also includes at least one area (collection chamber) in which gravitational forces act to trap particulate matter exhausted through the passageway. PFC gases exhausted through the passageway are subjected to RF power applied to the electrodes and excited into a plasma state. Constituents from the plasma react with silicon residue particles trapped in the collection chamber to convert the PFC gases into non-PFC gaseous byproducts. These and other embodiments of the present invention, as well as its advantages and features are described in more detail in conjunction with the text below and attached figures. Fig. 1 illustrates one embodiment of a simplified chemical vapor deposition apparatus to which the apparatus of the present invention may be attached; Fig. 2 illustrates one method of connecting the present invention to the chemical vapor deposition apparatus of Fig. 1; Fig. 3 illustrates a second method of connecting the present invention to the chemical vapor deposition apparatus of Fig. 1; Fig. 4(a) is a perspective view of a preferred embodiment of the apparatus of the present invention optimized for particle reduction (vacuum line cleaning) without a door, Fig. 4(b) is a front view of the vacuum line cleaning apparatus shown in Fig. 4(a) without a door; Fig. 4(c) is a front perspective cross-sectional view of the vacuum line cleaning apparatus shown in Fig. 4(a) taken from a plane in the center of the apparatus; Fig. 4(d) is a side perspective cross-sectional view of the vacuum line cleaning apparatus shown in Fig. 4(a) taken from a plane in the center of the apparatus; Fig. 4(e) is a cross-sectional view of the power feed through connection to the vacuum line cleaning apparatus shown in Fig. 4(a); Fig. 4(f) is a perspective view of the vacuum line cleaning apparatus shown in Fig. 4(a) that includes the door; Fig. 5 is an electrical diagram of the circuit connected to and including the electrodes shown in Fig. 4(a); Fig. 6 is a graph showing the effect of an electrostatic collector in one embodiment of the vacuum line cleaning apparatus of the present invention on particles generated by an exemplary silicon nitride deposition step; Fig. 7 is a graph showing the effect of electrostatic, gravitational and thermophoretic forces as compared to the neutral drag force on particles within one embodiment of the vacuum line cleaning apparatus of the present invention; Fig. 8 is a micrograph showing the amount of residue build-up on a silicon piece inside the vacuum foreline after a 15 second silicon nitride deposition process; Fig. 9 is a micrograph showing the elementary grain size of one grain of the residue shown in Fig. 8; Fig. 10 is a micrograph showing the size of particulate matter deposited on a silicon piece inside the vacuum foreline during an experiment performed prior to testing the present invention; Fig. 11(a) is a side cross-sectional view of another embodiment of the vacuum line cleaning apparatus of the present invention; and Fig. 11(b) is a perspective view of the vacuum line cleaning apparatus shown in Fig. 11(a); The apparatus of the present invention can be used in conjunction with a variety of different semiconductor processing devices. One suitable device, a chemical vapor deposition machine, is shown in Fig. 1 which is a cross-sectional view of a simplified, parallel plate chemical vapor deposition system 10. System 10 includes a gas distribution manifold 11 for dispersing deposition gases to a wafer, not shown, that rests on a susceptor 15 in a vacuum chamber 15. Susceptor 15 is highly thermally responsive. Susceptor 15 (and the wafer supported on the upper surface of susceptor 15) can be controllably moved between a lower loading/off-loading position and an upper processing position 11 which is closely adjacent manifold 11. When susceptor 15 and the wafer are in processing position 11, they are surrounded by a baffle plate 17 having a plurality of spaced holes 23 which exhaust into an annular vacuum manifold 24. During processing, gas inlet to manifold 11 is uniformly distributed radially across the surface of the wafer as indicated by arrows 21. The gas is then exhausted via ports 23 into the circular vacuum manifold 24 and through a vacuum foreline 31 by a vacuum pump system 32. Before reaching manifold 11, deposition and carrier gases are supplied through gas lines 18 into a mixing chamber 19 where they are combined and then sent to manifold 11. A controlled plasma is formed adjacent to the wafer by RF energy applied to manifold 11 from RF power supply 25. Gas distribution manifold 11 is an RF electrode, while susceptor 15 is grounded. RF power supply 25 can supply either single or mixed frequency RF power (or other desired variations) to manifold 11 to enhance the decomposition of reactive species introduced into chamber 15. A circular external lamp module 26 provides a collimated annular pattern of light 27 through quartz window 28 onto an annular outer peripheral portion of susceptor 15. Such heat distribution compensates for the natural heat loss pattern of the susceptor and provides rapid thermal and uniform susceptor and wafer heating for effecting deposition. A motor, not shown, raises and lowers susceptor 15 between a processing position 11 and a lower, wafer-loading position. The motor, gas supply valves (not shown) connected to gas lines 18 and RF power supply 25 are controlled by a processor 34 over control lines 36 of which only some are shown. Processor 34 operates under the control of a computer program stored in a computer-readable medium such as a memory 38. The computer program dictates the timing, mixture of gases, chamber pressure, chamber temperature, RF power levels, susceptor position, and other parameters of a particular process. Typically, any or all of the chamber lining, gas inlet manifold faceplate, support fingers 13, and various other reactor hardware is fabricated from material such as anodized aluminum. An example of such a PECVD apparatus is described in US-A-5,000,113. The above reactor description is mainly for illustrative purposes, and the present invention may be used with other CVD equipment such as electron cyclotron resonance (ECR) plasma CVD devices, induction coupled RF high density plasma CVD devices, or the like. The present invention may also be used with thermal CVD devices, plasma etching devices, physical vapor deposition devices and other substrate processing devices. The apparatus of the present invention and the method for preventing deposition build-up within a vacuum line is not limited to any specific semiconductor processing apparatus or to any specific deposition or etching process or method. During semiconductor processing operations such as chemical vapor deposition processes carried out by CVD reactor 10, a variety of gaseous waste products and contaminants are exhausted from vacuum chamber 15 into vacuum line 31. Depending on the particular operation being performed, these exhaust products may include either or both particulate matter, such as partially reacted products and byproducts that leaves a residue or similar powdery material within the foreline as it is exhausted through the foreline, or PFC gases. The present invention prevents build-up of such particulate matter in the foreline and/or reduces PFC gases emitted from vacuum chamber 15. Different embodiments of the present invention can be specifically designed and optimized to either prevent such particle build-up or reduce PFC gas emissions. Also, some embodiments of the present invention may be optimized to reduce both particle build-up and PFC emissions. As shown in Fig. 2, which is a cross-sectional view of the simplified CVD apparatus of Fig. 1 fitted with the apparatus of the present invention, the apparatus of the present invention is positioned downstream from the exhaust gas source -- the processing chamber. The apparatus may either connect to or replace a portion of the vacuum foreline. In Fig. 2, a downstream plasma cleaning apparatus 40 (hereinafter referred to as "DPA 40" or "the DPA") is fitted between vacuum pump system 32 and vacuum manifold 24 along a portion of vacuum line 31. Because of its position, gases exhausted from vacuum chamber 15 necessarily passes through DPA 40. DPA 40 may be positioned at any location along vacuum line 31, but preferably, DPA 40 is positioned as close as possible to exhaust manifold 24 so that gases exhausted from chamber 15 pass through DPA 40 before passing through any portion of vacuum line 31. Also, as shown in Fig. 3, it is possible to connect two or more DPAs to vacuum line 31. Such a configuration might be used, for example, to employ two DPA's optimized for particle collection to further protect vacuum pump 32 from particle and residue build-up. In the configuration shown in Fig. 3, a second DPA 42 is positioned downstream from DPA 40 just before pump 32. If any particulate matter escapes DPA 40, the matter can be trapped and converted into gaseous form within DPA 42. DPAs 40 and 42 can both be driven by a single RF power supply 44 with the power being split by a splitter 46. Optionally, DPAs 40 and 42 may each be driven by separate RF power supplies or may both be driven from the main RF power supply connected to processing chamber 10. Such a two-DPA configuration may also be used to employ two DPAs optimized toward PFC reduction to further limit emitted PFC gases. Or, alternatively, a two-DPA configuration can include one DPA optimized toward particle reduction and one DPA optimized toward PFC reduction. If separate DPAs optimized for PFC reduction and particle collection, are employed, it is preferable to position the DPA optimized for particle collection in the foreline upstream of the PFC reduction DPA. Such a configuration helps better prevent particle build-up in the entire foreline rather than just a latter section and can also reduce potentially unwanted particle build-up in the PFC-reducing DPA. Details of various configurations and embodiments of DPA 40 as configured and optimized to reduce particle build-up in the foreline and/or to reduce PFC emissions are discussed below. These embodiments are illustrated for exemplary purposes only. In no way should it be construed that the present invention is limited to these specific configurations or embodiments. Some embodiments of the present invention are configured and optimized for reducing particle and residue build-up within the foreline as such particles are exhausted from the chamber. As an example of such particulate matter, during the deposition of a silicon nitride film using silane (SiH 4 ), nitrogen (N 2 ) and ammonia (NH 3 ) as precursors, residue in the form of a brown powder composed of Si x N y H z , Si x H y and elemental silicon has been observed in the foreline. It is believed that this residue build-up is from half-reacted byproducts of the reaction of SiH 4 + N 2 + NH 3 . As far as the inventors are aware, silane-based silicon nitride CVD deposition operations are among the substrate processing operations that generate the most particles. Other substrate processing operations may also generate particle build-up and residue, however. For example, similar residues are also formed during the deposition of silicon nitride layers using other precursor gases or liquids such as disilane (Si 2 H 6 ) or organic sources. Residue build-up may also occur during the deposition of oxynitride films, silicon oxide, silicon carbide and amorphous silicon films among other layers and may also occur during plasma etching and other process steps. The particle reduction embodiments of the invention prevent build-up of such residues and particulate matter by trapping the particulate matter in a collection chamber and exciting reactant gases exhausted through the vacuum foreline and the residual and particulate matter within the collection chamber into a plasma state. The plasma reacts with the residue and particulate matter that is trapped in the collection chamber to form gaseous products and byproducts that may be pumped through the DPA and the vacuum line without forming deposits or condensing within the line. In operation, as deposition gases are exhausted from vacuum chamber 15 through vacuum line 31, particulate matter and residue from the gases are deposited on the interior surface of a gas passageway within the DPA. Removal of the particulate matter and residues may be achieved by activating DPA 40 to form a plasma within the DPA. The DPA is activated to form such a plasma during a clean cycle when etchant gases are exhausted from chamber 15. When activated, DPA 40 creates a voltage field that forms a plasma from the exhaust (etchant) gases passing through the DPA into a plasma state. The plasma enhances decomposition of the particulate and residual matter within DPA 40 into gaseous products and byproducts that may be pumped out through the foreline thus preventing particle deposition or residue build-up within the foreline. For example, in the case where residue build-up within DPA 40 is in the form of the brown powder comprising Si x N y H z , Si x H y and elemental silicon as described above in respect to silicon nitride deposition and the etchant gas employed during the clean cycle is a CF 4 and N 2 O mixture, it is believed that the plasma formed by DPA 40 breaks the residue down into gaseous components such as SiF x , COF 2 , F 2 , SiOF 2 , CO and CO 2 , NO, O and O 2 . In some applications, rather than creating a plasma from etchant gases exhausted into the DPA, DPA 40 actually maintains the plasma formed in the substrate process chamber. That is, in these applications, some or all of the plasma formed in the chamber may still be active downstream of the chamber. This may occur, for example, during a chamber clean operation when a plasma is formed from highly reactive fluorinated species. Constituents from the plasma may be exhausted from the chamber into the foreline and DPA while still in an excited or plasma state. Thus, in these embodiments, the voltage field in DPA 40 may actually maintain a plasma rather than form a new one. Neither the design nor the operation of the DPA needs to be modified depending on whether a plasma is maintained or created within the DPA. While DPA 40 is activated to form and/or maintain a plasma only during clean cycles in most embodiments, it is possible in other embodiments to sustain a plasma during both deposition and clean cycles in order to further react the CVD gases in other embodiments. In such a configuration, an additional etchant gas can be introduced upstream of the DPA or directly into the DPA during the deposition cycle as described in more detail below. In addition to collecting residue by normal deposition within DPA 40, various preferred embodiments of DPA 40 are specifically designed to trap particulate matter exhausted from chamber 15 within the DPA so that the matter cannot be deposited downstream of the DPA. Trapping is done using mechanical, electrostatic and/or thermophoretic trapping mechanisms as described in more detail below. Once trapped, particulate matter remains in DPA 40 until it reacts during the clean process with active species in the plasma to form gaseous byproducts that are then pumped through vacuum line 31. In these embodiments, it is possible to effectively reduce particle build-up without the application of a voltage field that forms or maintains a plasma within the DPA. This is possible, for example, when the ionization of the etchant (e.g., fluorine) is sufficiently high during the chamber clean such that the free radicals generated in the clean plasma have a lifetime long enough to still be in an excited state when exhausted into the DPA. In such an excited state, the free radicals can react with the trapped particulate matter and convert the matter to gaseous products as described above. A plasma can be generated within DPA 40 using a variety of known techniques such as application of HF or RF power to capacitively coupled electrodes or inductively coupled coils or by microwave or ECR techniques. Specific embodiments of some of these methods are described below in more detail. In each embodiment described below, it is preferable for cost efficiency reasons that the described DPA be designed to be process transparent. That is, DPA 40 is preferably designed to prevent particle build-up within the foreline without requiring the use of any extra clean gas or extra clean time. Also, it is preferable that the DPA not have any adverse effect on film properties such as uniformity, particle contamination, stress and others. Figs. 4(a)-4(f) are various perspective and cross-sectional views of the preferred embodiment of DPA 40 configured and optimized for the abatement of residue and particle build-up. Fig. 4(a) is a front perspective view of DPA 40 having its door removed; Fig. 4(b) is a front plane view of the DPA (with the door removed); Fig. 4(c) is front perspective cross-sectional view taken along a plane in the center of the DPA; Fig. 4(d) is a side perspective cross-sectional view taken along a plane in the center of the DPA; Fig. 4(e) is a cross-sectional view of a power feed through connection to DPA 40; and Fig. 4(f) is a perspective view of DPA 40 having a door and handles attached. As shown in Figs. 4(a) to 4(f), DPA 40 includes an inlet 50 and an outlet 52 (see Fig. 4(c)). In between inlet 50 and outlet 52 is a fluid conduit 54 (gas passageway) that is defined by a pair of opposing aluminum electrodes: cathode 56 and anode 58 (see Fig. 4(a)). DPA 40 is connected to the foreline (or directly connected to the processing chamber) through coupling mechanisms 64 and 66 (Fig 4(a)). For example, in one embodiment DPA 40 is connected directly to the chamber exhaust port by coupling mechanism 64 and the beginning of the foreline is connected to the DPA at coupling mechanism 66. Gases and particulate matter exhausted into the foreline from the substrate processing chamber pass into DPA 40 through inlet 50 and exit from outlet 52. A removable aluminum door 63 (Fig. 4(d)) encloses gas passageway 54 along with a backplate 65 (Fig. 4(d)). Aluminum door 63 and backplate 65 are electrically coupled to electrode (anode) 58. Electrodes 56 and 58, door 63 and backplate 65 form an enclosed vacuum chamber (fluid conduit 54) that prevents gases exhausted into DPA 40 from escaping. Door 63 and backplate 65 each include a ceramic insulation plate 71 (Fig. 4(d)), which contacts electrodes 56 and 58 to form a seal that prevents gases exhausted through the DPA from migrating outside a gas flow path indicated by arrows 60 (Fig. 4(b)). In a preferred embodiment, a Teflon cushion 73 (Fig. 4(d)) is included in door 63 between the aluminum door and the ceramic insulation layer 71. Teflon cushion 73 has a higher thermal expansion than ceramic insulation layer 71 and is also relatively soft, which allows it to expand without breaking or cracking. When DPA 40 is activated to form a plasma, heat is generated that causes Teflon layer 73 to expand and press ceramic insulation layer 71 against electrodes 56 and 58. This helps ensure an adequate seal at door 63 so that gases do not escape from the DPA. Door 63 is attached to DPA 40 through screws 59 (Fig. 4(f)) and can be removed with handles 67 (Fig. 4(f)) by removing the screws. Once removed, the interior of DPA 40 may be cleaned or hand wiped with a wet solution such as alcohol and/or vacuumed to remove particle build-up or residue that may occur after extended use or for other reasons. In a preferred embodiment, handles 67 are made from a poor heat conducting material such as plastic. Electrodes 56 and 58 are electrically separated from each other by four insulative plugs (made from ceramic in a preferred embodiment) 61 (Fig. 4(a)), which are also referred to as cathode retainers. As shown in the Figs., electrodes 56 and 58 have grooves machined in them to house a portion of the cathode retainers. Two cathode retainers 61 are shown in the drawings on the front side of the DPA and the other two are similarly positioned on the backside of the DPA. In one embodiment, cathode retainers 61 are each approximately 1 cm thick. Thus, cathode retainers 61 do not extend the entire width of gas passageway 54 and do not block the gas flow through the passageway. In the DPA, gas flow follows fluid conduit 54 as indicated by arrows 60 (Fig. 4(b)). Fluid conduit 54 includes two mirror image gas flow paths. An extruding portion of cathode 56 (flow divider 57 -- see Fig. 4(b)) directs the effluent gas into one of the two flow paths. Approximately half the gas flow is diverted toward the passage on the left side of DPA 40 while the other half is diverted toward the right side of the device. Fluid conduit 54 is a labyrinthal/serpentine passageway that includes particle collection areas 62 (Fig. 4(a)) that rely in part on gravitational forces to collect and trap particles present in the effluent gas stream, e.g., particles generated during a substrate deposition or other type of processing step. Each particle collection area 62 is a "U"-shaped segment of the gas passageway that is situated so that particles are collected and held within the bottom area of the "U" portion by gravitational forces despite an effluent gas flow path that attempts to drag the particles out of the DPA. Gas flow is directed through each "U" portion by an extruding finger 79 of either cathode 56 or anode 58 as shown in Fig. 4(c). These particle collection areas 62 are collectively referred to as a gravity or mechanical trap and are discussed in more detail below. Electrodes 56 and 58 form both a parallel plate plasma generation system and an electrostatic particle collector. As part of an electrostatic particle trap, DC power is applied to electrode 56 while electrode 58 is grounded to attract exhausted particulate matter which is electrically charged. The applied DC power creates a voltage field that attracts positively charged particles exhausted through the DPA on one electrode and attracts negatively charged particles on the other. Being grounded, electrode 58 also acts as a Faraday cage for RF shielding. As part of a plasma generation system, RF power is applied to electrode 56. The applied RF power forms a plasma from effluent gases passing through the DPA and etches away particles and residue collected in either the gravity trap areas 62 or along the surface of electrodes 56 and 58. Fig. 5 is a diagram showing the electrical circuit that includes electrodes 56 and 58. As shown in Fig. 5, electrode 56 is connected to both a DC generator 100 and an RF generator 102 while electrode 58 is grounded. DC generator 100 supplies the DC voltage required by the electrostatic trap, and RF generator 102 supplies the RF power to form a plasma. An RF match circuit 104 matches the generator output impedance to 50 W to minimize the reflected power, and a DC/RF filter (a low pass RC filter) 106 isolates DC power supply 100 from the RF signal interferences. RF generator 102 can be the same power supply as RF power supply 25 shown in Fig. 2 or can be a separate RF power supply that drives only DPA 40. Additionally, assuming multiple processing chambers are present in a clean room, the multiple DPAs connected to the chambers may all be driven by a separate, dedicated DPA RF power supply that is connected to an appropriate number of RF power splitters. To ensure complete reaction of material passing through and/or deposited within DPA 40, the DPA must be driven by the RF power supply (e.g., RF generator 102) at a level sufficient to form and/or maintain a plasma. Generally, a power level of between 50-2000 Watts or more can be employed depending on the surface area of the cathode and the desired intensity of the plasma. In an embodiment where the surface area of cathode 58 is about 774 cm 2 (120 in 2 ), preferably a power level of between 750-1000 Watts (between 0,98 and 1,31 W/cm 2 (6.31 and 8.42 W/in 2 )) is used. The actual power level selected should be determined by balancing a desire to use a high power level to form an intense plasma and a desire to use a low power level to save energy costs and allow use of smaller, less expensive power supplies. The power supply driving DPA 40 is operated at a frequency range from about 50 KHz to about 200 MHz or more and is preferably operated in the range of about 50 KHz to 60 MHz. Generally, lower frequency power supplies are less expensive to purchase and operate than higher frequency supplies. Accordingly, in the most preferred embodiments, the power supply driving DPA 40 is designed to provide RF frequency of 325 KHz or less. RF power supply can be supplied from either a single frequency RF source or a mixed frequency RF source. The optimal power output and operating frequency of the supply will depend on the application for which the DPA is used and on the volume of the gas to be treated in DPA 40 along with cost considerations. Electrical connections to DPA 40 are made through a power feed through piece (PFD) 68 (Fig. 4(a)). PFD 68 is shown in detail in Fig. 4(e), which is an enlarged side elevational view of PFD 68. PFD 68 connects DC generator 100 and RF generator 102 to cathode 56 through a connector 70. In a preferred embodiment, connector 70 is a threaded screw that screws directly into cathode 56. To reduce corrosion at the RF connection and to maintain an adequate electrical connection between screw 70 and cathode 56, the connection should be made at atmospheric pressure. This area of atmospheric pressure is shown as area 76 and includes the area of the threaded screw 70 that contacts cathode 56. An o-ring 78 maintains a seal between cathode 56 and area 76. To prevent o-ring 78 from melting from the intense heat that may be generated during operation of the DPA, a specially designed area is provided to reduce heat transfer from the main portion of cathode 56 (shown as area 56A) to the area of cathode 56 (shown as area 56B) in which o-ring 78 is embedded. This specially designed area includes vacuum areas 80 and a thin portion 82 of cathode 56. Heat generated and/or transferred to area 56A of cathode 56 does not readily transfer to area 56B because vacuum areas 80 mostly isolate cathode area 56B from cathode area 56A. The small portion of cathode 56 (portion 80) that transfers the RF and DC signals from area 56B to area 56A is sufficiently thin to significantly reduce the heat transferred from area 56A to area 56B. The power feed through connection is housed in an aluminum housing 72 and insulated from housing 72 and door 63 by teflon plate 73 and teflon rings 74, 75 and 81. Housing 72 is electrically connected to anode 58 and door 63. A flat washer 84, lock washer 85 and nut 86 assembly allows clamping of teflon ring 75 and teflon lining 73 to area 56B of cathode 56. This clamping force compresses o-ring 78 to maintain an adequate seal. A second o-ring, o-ring 77, maintains a seal between teflon lining 73 and door 63 so as to not allow escape of gases through power feed through connection 68. In standard operation, DC power is supplied to electrode 56 during substrate processing steps, such as a CVD step, to enhance the particle trapping capabilities of DPA 40. The voltage applied to electrode 56 varies depending on the application. Typically, the application of between 100-3000 volts creates an effective trapping mechanism. Such DC voltage can be applied at all times during chamber operation (processing and clean steps) or may be stopped during the chamber clean operation when DPA 40 is activated. In one substrate processing operation, where silicon nitride was deposited from a process gas of SiH 4 , N 2 and NH 3 experiments determined that approximately 60% ± 10% of the generated particles were positively charged while approximately 40% ± 10% of the generated particles were negatively charged. As shown in Fig. 6, experiments determined that the creation of approximately a 500 volt/cm DC field within DPA 40 provided an optimal electrostatic collector for use with this substrate processing operation. In Fig. 6, line 110 represents the total accumulation of negatively charged particles collected on the positively charged electrode from the creation of between a 200-1200 volt/cm electric field between the electrodes, and line 112 represents the total accumulation of positively-charged particles collected on the grounded electrode. Line 114 represents the total accumulation of trapped particles. At voltage fields lower than 500 volts, larger particles may not be effectively trapped by the electrostatic collector, while the creation of higher voltage fields forms a partial plasma. Such plasma formation changes the characteristics of the generated electric field and degrades trapping efficiency. The electrostatic collector and mechanical (gravity) trap combination provides a particularly effective mechanism to prevent deposition build-up in vacuum line 31. The gravity trap is particularly effective in trapping relatively large particles present in the effluent gas stream because these particles are more likely to be held within exterior tube 62 by gravitational forces. The electrostatic trap, on the other hand, is particularly effective at collecting and trapping smaller particles in the effluent gas stream that may otherwise not be collected by just the gravity trap. As an example, in the deposition of silicon nitride as described above, particles ranging in size from 1 mm in diameter to 1 mm in diameter or more have been observed. When these particles are in the exhaust line, two forces of importance act on the particles; a gravitational attraction force (F g ) and a neutral drag force (F nd ) resulting from the gas motion. For large particulate matter, such as particles larger than 100 mm in diameter, the major interaction is the gravitational force, so the mechanical trap is particularly effective. For smaller particles, however, the drag force of the gas can be higher than the gravitational force. Consequently, the electric field developed between the two electrodes of the electrostatic trap applies a supplementary force (F elec ), perpendicular to the trajectory of the particulate. This force can be two or more orders of magnitude larger than both the gravitational and drag forces for very small particulates, such as those less than 10 mm in diameter, resulting in a very high collection efficiency. Fig. 7 is a graph showing the effect of the electrostatic and gravitational forces as compared to the neutral drag force on particles according to one embodiment of the present invention. Line 152 shows the gravitational force, line 154 shows the electrostatic force and line 156 shows the particle neutral drag force. As shown, for smaller particles, electrostatic force 154 is greater than the gravitational force 152. For larger particles, gravitational force 152 is dominant over electrostatic force 154. In this embodiment, the forces are such that particles up to about 30 micrometers in diameter are primarily collected by the electrostatic collector, while particles larger than about 30 micrometers are primarily collected by the mechanical trap. Regardless of whether the electrostatic or gravitational force dominates for any given particle, the focal point of Fig. 7 is that DPA 40 is preferably designed so at least one of electrostatic force 154 or gravitational force 152 should be larger than the neutral drag force 156 for a particle of any given size. In such a case, the combination of electrostatic and mechanical trap collectors, ensures that particles of various sizes will be effectively collected. A fourth force, the thermophoretic force (F th ), also acts on particles within DPA 40. The thermophoretic force is due to temperature gradients created within the DPA. Such temperature gradients may be created, for example, by formation of a plasma during the plasma-assisted clean process. During plasma formation cathode 56 becomes hotter than anode 58 because of ion bombardment and the Joule effect during plasma formation. In one embodiment, the temperature gradient between cathode 56 and anode 58 is 200°C/cm at a gas temperature of 150°C. The thermophoretic force in this embodiment is shown in Fig. 7 as line 158. While thermophoretic force 158 may not be strong enough to trap particles of between 0.1 to 100 µm in this embodiment, it can contribute to the trapping of both charged and uncharged particles. Also, in other embodiments, a person of ordinary skill in the art would understand how to create larger temperature gradients such that a larger thermophoretic force is created and to more effectively aid in particle and residue entrapment. As previously stated, during a chamber clean operation RF energy is applied to electrode 56 to form and/or maintain a plasma from effluent etching gases exhausted into the DPA. Constituents from the plasma react with the particles and residue trapped within the DPA from one or more previous substrate processing steps. Preferably, the application of RF energy to form this plasma is discontinued during times when etchant gases are not exhausted through the DPA (in such a configuration DPA 40 is referred to as an active, rather than a passive device). Control of the timing aspects of DPA 40 (e.g., switching RF power supply 102 and/or DC power supply 100 ON and OFF) when DPA 40 is configured as an active device is generally performed by processor 34 through the application of control signals sent over control lines 36 shown in Fig. 1. Even though not shown in Fig. 15, such control lines are connected to DPA 40 in such a configuration. In an alternative embodiment, it is possible to provide a gas supply line directly to DPA 40 for the introduction of an etchant gas separate from and in addition to the etchant gas exhausted from chamber 15 during a clean operation. Such an extra gas supply line can be connected to the DPA, for example, at or near inlet 50. It could also be connected directly to the foreline at a position upstream from the DPA. If such a separate gas line is provided, the additional supply of etchant gas can be supplied to the DPA during a clean sequence only, during a deposition or other substrate processing step only, or it could be continuously supplied during both deposition and clean cycles. In an embodiment where the etchant gas is supplied to the DPA during a substrate processing step, RF energy is applied to electrode 56 during the substrate processing step to form a plasma and further etch deposited material from within the DPA. The effectiveness of DPA 40 in trapping particles and reducing deposition build-up depends on a number of factors including the amount of particles generated and exhausted from chamber 15, the rate of effluent gas flow through DPA 40, the voltage field created between electrodes 56 and 58, the surface area of electrodes 56 and 58 and the intensity of the plasma generated during the clean phase among others. Also, a number of other design considerations enhance the effectiveness of DPA 40. For instance, in preferred embodiments, the upper surface of flow divider 57 (Fig. 4(a)) is sharply angled to a single edge. Experiments showed that deposition build-up collects more rapidly at locations where gas flow directly contacts a barrier or other surface within the DPA. The angled surface of flow divider 57 combined with the introduction of the effluent gas through inlet 50 directly above and perpendicular to the single edge of flow divider 57 provides a smaller area of contact for the effluent gas stream entering DPA 40 through inlet 50 and thus minimizes deposition on the upper surface of flow divider 57. In experiments performed without such an angled surface (e.g., a rounded surface), particle build-up collected on the upper surface of divider 57. Depending on the amount of such build-up, it is possible for the build-up to break off and fall into one of collection areas 62. If the particle build-up is sufficiently large, it may not be dissipated by the plasma formed during the normal clean cycle. This could lead to blockage of the gas passageway. Also, if the build-up is dielectric material (e.g., build-up from silicon nitride deposition), the build-up interferes with plasma generation and lessens the strength of the formed plasma. This in turn results in less etching of deposited material and an increased chance of passage blockage. Preferably, the side surfaces of flow divider 57 meet at an angle of 30 degrees or less to prevent such build-up. It is even more preferable that the formed angle be about 10 degrees or less. Another design feature that lessens particle build up in any one particular area of DPA 40 is the contour of the walls of the portion of gas passageway 54 between inlet 50 and the point at which gas flow is divided into left and right flows. A smooth, contoured transition (or flare out) at inlet 50, as opposed to one with sharp angles, into the gas passageway helps ensure an even distribution of the gas flow into the passageway. This contoured transition from inlet 50 to fluid conduit 54 is referred to as a profiled manifold. An even gas flow through the profiled manifold helps ensure an equal distribution of gas flow into each of the left and right portions of gas passageway 54 thereby preventing a greater particle build up in one portion of the passageway as compared to the other. The profiled manifold also ensures an even gas distribution throughout the entire width of the electrodes. The contour of a preferred version of the profiled manifold is shown in detail in Figs. 4(c) and 4(d) as gas passage surfaces 55. Formation of a uniform plasma helps ensure complete removal of particles and residue collected within DPA 40. To this end, it is useful for the surface area of electrode 56 be approximately the same as the surface area of electrode 58. Experiments performed in a DPA having a difference in surface area between electrodes of between 3:1 and 1.3:1 at various locations indicate that it is possible to form a plasma with uneven surface area electrodes and that such a plasma can adequately remove some material collected within the DPA. In these experiments, however, particle and residue build up was more effectively removed in the area of the DPA where electrode surface area ratio was closer to 1.3:1 rather than 3:1. In further experiments in which the surface area of cathode 56 was within 95 percent of the surface area of the anode (766,2 vs. 980,75 cm 2 (118.79 in 2 vs. 153.31 in 2 )) 58, plasma formation was more intense and particle removal even more effective. In other embodiments the surface area of the cathode is substantially equivalent to that of the anode. Another plasma uniformity issue involves the spacing of electrode 56 relative to electrode 58. This spacing should basically remain constant throughout the gas passageway of DPA 40 with the following exception. The voltage breakdown of a plasma is a function of the pressure and distance between electrodes (P x D). For the effluent gas stream to flow through DPA 40, it is necessary that the pressure near inlet 50 be slightly higher than the pressure near outlet 52. To keep the voltage breakdown constant in this preferred embodiment, more space is introduced between the electrodes in the lower portion of DPA 40 than between the electrodes in the upper portion. This spacing variation can be done by, for example, making the protruding fingers of either or both electrode 56 and/or electrode 58 thicker in the upper portions of the DPA as shown in Fig. 4(b). In Fig. 4(b), fingers of cathode 56 and anode 58 in the upper portion of DPA 40 have thicknesses of a and b, respectively. While the corresponding parts in the lower portion of DPA 40 have thicknesses of c and d, respectively, where a > c and b> d. Pressure within the DPA also effects plasma formation. Generally, a higher pressure results in a more efficient plasma etching. Thus, operating the DPA at a higher pressure allows lower power requirements than a lower pressure, which in turn results in operating cost savings. A higher DPA pressure can be obtained by positioning a throttle valve in the foreline after the DPA. In such a configuration it is possible to use a single throttle valve downstream from the DPA or preferably dual throttle valves: one upstream of the DPA to control chamber pressure and one downstream to control DPA pressure independent of the pressure within the processing chamber. Without a throttle valve downstream of the DPA, the pressure within the DPA is generally equal to the pressure of the foreline between about 106.6 Pa to 333.3Pa (0.8 to 2.5 torr) in some PECVD processing apparatuses operated at about 600 Pa to 800 Pa (4.5 to 6 torr). With a throttle valve downstream from the DPA, however, pressure within the DPA can be controlled over a broader range. Of course pressure in the DPA must be less than the pressure within the chamber to maintain an effluent gas stream from the chamber. Increasing the pressure within the DPA also has an unwanted side effect of increasing the neutral drag force of particles exhausted into the DPA, which in turn, reduces the efficiency of the gravitational trap. Thus, the actual pressure set in the DPA should balance plasma efficiency considerations with particle trapping considerations and will depend on the particular application the DPA is employed in. A pressure sensitive switch 53 (Fig. 4(d)) can be included to monitor the pressure within DPA 40. In the event that pressure within the DPA builds-up to undesirable levels, switch 53 sends a signal to processor 34 to turn OFF both the DPA and substrate processing chamber 10. In a preferred embodiment, switch 53 is a half-atmosphere switch that initiates the shutdown procedure when the pressure within DPA 40 increases beyond one half atmosphere 47988 Pa (360 torr). Depending on the RF power used to form a plasma within the DPA, the size of the cathode, the time period over which the DPA is activated and other factors, DPA 40 can generate a substantial amount of heat. To dissipate the heat, DPA 40 can include heat dissipation fins 69 as shown in Fig. 4(e). Heat dissipation fins 69 are attached to anode 58. Heat is generated during plasma formation at cathode 56 by ion bombardment and the Joule effect. Consequently, anode 58 is cooler than cathode 56. Additionally, anode 58 is thermally insulated from cathode 56 by ceramic cathode retainers 61, ceramic lining plate 71 (at the backside and door) and the TEFLON insulator rings in PFD 68. Fins 69 help further cool the anode. Fins 69 are constructed out of a heat conducting material such as aluminum and are the preferred method of cooling DPA 40 in that they are a passive cooling device. For safety reasons it is preferable to design fins 69 so that the exterior of DPA 40 is cooled to at least 75°C or below. In a preferred embodiment in which the DPA is outfitted for a DCVD chamber in a P5000 reactor system manufactured by Applied Materials, the fins are positioned on three sides of the DPA, but not on a fourth side. Instead, the fourth side (the backside) of the DPA is placed directly against part of the substrate processing chamber. The degree of cooling provided by fins 69 depends on the size of the fins. In one embodiment where the temperature of the cathode runs between 250-300°C, fins 69 are sufficiently large to cool the exterior of the DPA to about 75°C. It is also possible to use other methods to cool DPA 40. For example, a cooling system that circulates water around DPA 40 can be used to transfer heat away from the DPA. Such a cooling system is an active cooling mechanism. To prove the effectiveness of the present invention in reducing particle build-up, experiments were performed in which a DPA 40 designed according to the preferred embodiment described above was attached to a Precision 5000 chamber outfitted for 15,24 cm (6 inch) wafers and designed for CVD deposition of silicon nitride. The Precision 5000 chamber is manufactured by Applied Materials, the assignee of the present invention. Before experiments were performed testing the effectiveness of the DPA, experiments were performed to determine the composition of residue deposited in the processing chamber by a silicon nitride deposition step followed by a fluorine clean step. The composition of the residue was determined for two different silicon nitride deposition/fluorine clean operations process sequences. In each process sequence, the silicon nitride deposition step was identical while the clean step was based on a CF 4 chemistry in the first sequence and on an NF 3 chemistry in the second sequence. The silicon nitride film was deposited on a wafer by subjecting the wafer to a plasma of silane (SiH 4 ), nitrogen (N 2 ) and ammonia (NH 3 ) gases. SiH 4 was introduced into the chamber at a flow rate of 275 sccm, N 2 was introduced into the chamber at a rate of 3700 sccm and NH 3 was introduced at a rate of 100 sccm. The plasma was formed at a pressure of 600 Pa (4.5 torr), at a temperature of 400°C, using a 13.56 MHz RF power supply driven at 720 Watts. The silicon nitride deposition process deposited lasted approximately 75 seconds which was sufficient to deposit a film of approximately 10,000 Å on the wafer. For the first sample, after the silicon nitride deposition step was completed and the wafer removed from the chamber, the chamber was cleaned with a CF 4 and N 2 O plasma for 150 seconds. The ratio of CF 4 to N 2 O was 3:1 with the CF 4 being introduced at a rate of 1500 sccm and N 2 O being introduced at a rate of 500 sccm. During the clean step, the chamber was maintained at a temperature of 400°C and at a pressure of 666.5 Pa (5 torr). The plasma was formed with a 13.56 MHz power supply powered at 1000 Watts. For the second sample, the chamber was cleaned with a plasma formed from NF 3 and N 2 O and N 2 precursor gases. The ratio of NF 3 to N 2 O to N 2 was approximately 5:2:10 with NF 3 being introduced at a rate of 500 sccm, N 2 O being introduced at a rate of 200 sccm, and N 2 being introduced at a rate of 1000 sccm. The chamber was maintained at a temperature of 400 °C and a pressure of 666.5 Pa (5 torr) during the clean step, which lasted for approximately 95 seconds. Plasma formation was achieved with a 13.56 MHz power supply powered at 1000 Watts. It was found that the color of the residue for the CF 4 clean was brownish while the color of the residue sample for the NF 3 clean was yellow/white. Residue produced from just an Si 3 N 4 deposition step was found to be brown, thus it is believed that these results indicate a more complete conversion of the initial brown powder into a yellow/white powder from the NF 3 clean. This is believed to be due to extra free fluorine radicals that are generated in the NF 3 plasma. In another series of experiments, three different residue samples were collected: powder collected in the foreline approximately 0.5 m downstream the processing chamber immediately after an Si 3 N 4 deposition step as described above (sample A); powder collected at the same location as sample A, but after running a NF 3 /N 2 O/N 2 clean plasma as described above (sample B); and powder collected at the inlet of a dry vacuum pump approximately 15 m downstream from the chamber after several days of continuous deposition/clean sequences (powder C). The composition of the powder samples was deduced from Hydrogen Forward Scattering (HFS), X-Ray Photoelectron Spectroscopy (XPS) and X-Ray Diffraction (XRD) analysis. The composition of each of these powders is shown in Table 1. Residue Formation From Silicon Nitride Deposition/Fluorine Clean Processes Sample Residue Type Residue Composition C at % O at % N at % Si at % F at % H at % A Brown powder 2.1 41 8.0 33 0.9 15 B yellow/white powder 0 1 12 8.5 38.5 40 C orange/brown powder 0.2 6.8 13 42 1 37 Sample A is the direct solid byproduct of the Si 3 N 4 deposition chemistry. The powder reflects the composition of the particles generated within the RF plasma. The powder is mostly composed of Si, N, H and oxygen. The oxygen is probably absorbed from air during sample collection. The oxygen cannot be an initial component of the powder because no oxygen-containing gas is employed during deposition. The residue particulates generated in the plasma are most likely highly hydrogenated silicon nitride Si x N y H z . This powder is highly reactive. XPS measurements confirm the HFS results and show that, after exposure to air, silicon is present 18% as elemental, 24% as nitride and 58% as oxide. Nitrogen is present 93% as nitride and 7% as ammonium. XRD analysis shows the powder to be amorphous. Sample B is the result of the conversion of powder A after a clean process using an NF 3 /N 2 O/N 2 plasma. The clean process vaporizes completely the residues accumulated inside the deposition chamber, but the conversion is not complete in the foreline, due to limited F* free radicals lifetime. However, this lifetime is long enough so that a partial conversion can happen in the first meters of the foreline. This white powder presents a high F content, accounting for the transformation of Si x N y H z into (NH 4 ) 2 SiF 6 (ammonium hexafluoro silicate, which has been identified using X-Ray Diffraction characterization). The polycrystalline white powder shows a sublimation temperature of 250°C. The amount of accumulated powder in sample B increases with an increase in the distance from the process chamber suggesting that the solid-gas vaporization becomes less and less efficient as gases are travelling along the foreline. This is probably due to the rarefaction of excited species such as F*, CF x , O*, among others while moving away from the chamber. Close to the pump, a mixture of powder A and B exists. This residue (powder C) is yellowish to brown as the distance from the chamber increases. Chemical analysis shows powder C is an incomplete conversion from powder A. Of interest is the formation of a polymer -(-CF 2 -C 2 F 4 -O-)x- that can coat the initial brown residues, forming a protective coating and preventing further conversion of the powder collected during deposition. The accumulation of powder C in the foreline of a commercial PECVD silicon nitride system can be more than 500 g after one month of continuous deposition/clean sequences. After the composition of residue build-up in the chamber was determined, an experiment was performed to determine the grain size of the residual powder. For this experiment, a silicon piece was placed within the foreline to collect material deposited there from the deposition process. It was observed that even after a 15 second deposition process, a residue build-up in the form of a brown powder normally occurs in vacuum line 3. A micrograph showing this residue build-up is shown as Fig. 8. The brown powder was made up of Si x N y H z , Si x H y , SiO x and elemental silicon residues. Elementary grains of the residue present a granular and spongy morphology, with a density of 2.3 g/cm 3 . The spheroid symmetry of the grains is shown in Fig. 9 and indicates that growth can be by homogeneous nucleation. Fig. 10, is a micrograph illustrating the aggregation of four or five elementary grains (15-20 µm in diameter each) into a typical residue aggregate particle that is approximately 50 µm in diameter. Further experiments showed that the grain size of the powder increased with deposition time to form aggregates 1.0 mm or larger in diameter for a 90 second deposition step. A prototype DPA was then employed to prove the effectiveness of the present invention in reducing particle build-up such as that from a silane-based silicon nitride deposition operation. The prototype DPA was constructed according to the embodiment shown in Figs. 4(a)-(f) and was connected to the exhaust port of a P5000 CVD chamber just before the foreline. In the experiment, the chamber was operated according to a common deposition/clean sequence for silicon nitride that included three sequential 1.0 micron deposition steps (for three separate wafers) followed by a CF 4 /N 2 O clean step. The deposition/clean cycle was continuously repeated for a 5000 wafer run test. For the silicon nitride deposition step, the chamber pressure was set and maintained at 600 Pa (4.5 torr), while chamber temperature was set to 400°C and the susceptor was positioned 1,5 cm (600 mils) from the gas distribution manifold. Deposition gases included SiH 4 introduced at a rate of 190 sccm, N 2 introduced at a rate of 1500 sccm and NH 3 introduced at a rate of 60 sccm. Single frequency RF power at a frequency of 13.56 MHz was supplied at a power level of 455 Watts to form a plasma and deposit the silicon nitride layer at a rate of approximately 7500 A/min. Total deposition time for each 1.0 micron layer was about 80 seconds. For the chamber clean step, chamber pressure was set and maintained at 613.2 Pa (4.6 torr), chamber temperature was set at 400°C and the susceptor (without a wafer) was positioned 1,5 cm (600 mils) from the gas distribution manifold. The clean gas included CF 4 introduced at a rate of 1500 sccm and N 2 O introduced at a rate of 500 sccm. RF power was supplied to form an etching plasma and etch away material deposited within the chamber. The frequency power supply was operated at 13.56 MHz and driven at 1000 Watts. Total clean time used to clean the chamber after three 1.0 micron silicon nitride layer deposition steps was 110 seconds for the first 3000 wafers. An endpoint detector was then used to optimize the clean time for the final 2000 wafers. The prototype DPA was approximately 35 cm in length by 11 cm in diameter. Electrodes 56 and 58 provided a total surface area of 1561,55 cm 2 (242.1 in 2 ) and were machined out of aluminum. The cathode was 7,62 cm (3.00 inch) wide and had a perimeter of 100,57 cm (39.5966 inch). A DC voltage of 500 volts was created between electrode 56 and electrode 58 to trap electrically charged particles during both the silicon nitride deposition step and the CF 4 clean step as described above. The voltage field was created by supplying 500 volts to electrode 56 and grounding electrode 58. As for plasma formation, the DPA device was operated as an active device (i.e., RF power was provided to the DPA to form a plasma during the clean cycle only - RF power was not provided during the deposition steps). Plasma formation was generated by a 325 KHz RF waveform driven at 1000 Watts. Pressure within the DPA was measured at 106.6 Pa (0.8 torr). In further testing, the prototype DPA was shown to be 100% effective in preventing particle build-up within the foreline during a 20,000 wafer test run using the above-described silicon nitride deposition/CF 4 clean sequence while being transparent to the process. The use of the prototype DPA did not require any additional clean gases or any additional clean time to trap and remove all particulate matter exhausted from the chamber during the experiment. Measurements taken of film characteristics such as thickness, uniformity, stress and refractive index for the deposited silicon nitride films during the experiment showed that there was no noticeable or significant change in any of these characteristics between the 1st and 5000th wafers (or between any wafers in between). Additionally, measurements taken measuring the particle count within the chamber during the experiment also showed there was no increase in particles of diameter 0.16 µm or greater during the wafer run. Fig. 11(a) is a cross-sectional view of another embodiment of DPA 40 that includes mechanical and electrostatic traps. The embodiment of Fig. 11(a) also includes evenly spaced, parallel electrodes 430 and 432 that form a capacitively coupled plasma from effluent gases exhausted into the DPA. Electrode 430 is connected to an RF and DC power supply and electrode 432 is grounded. Each electrode is formed from sheet metal and bent to create gas passageway 435. Effluent gas from chamber 15 enters the gas passageway through an inlet 434 and exits from an outlet 436. Gas passageway 435 includes two gas flow paths, path 435a and path 435b, that are positioned side-by-side and that are initially separated by a portion of electrode 430. Dividing the gas flow passage into two separate passageways in such a manner increases the surface area of electrodes 430 and 432 within a fixed area. Electrodes 430 and 432 should be sufficiently thick so that they do not melt and/or bend under the heat conditions generated by RF plasma formation within the DPA. In other embodiments, electrodes 430 and 432 can be machined out of aluminum. Fig. 11(b) is a perspective view of the DPA shown in Fig. 11(a). In Fig. 11(b), DPA 40 is encased by an aluminum casing 440 similar to other, already described embodiments of the DPA. Casing 440 includes a door 441 attached to the DPA by screws 442. DPA 40 can be cleaned by removal of this door. Also, RF and DC power is supplied to electrode 430 through a PFD connection 438.

Claims: Apparatus to be connected in an exhaust line of a substrate processing chamber for minimizing deposition in an exhaust line of the substrate processing chamber comprising a fluid conduit (54) having an inlet (50), an outlet (52) and a collection chamber between the inlet and the outlet, the collection chamber being formed by the fluid conduit (54) including at least one U-shaped portion for mechanically collecting particulate matter flowing through the fluid conduit and to inhibit egress of the particulate matter from the collection chamber, first and second electrodes (56, 58) each having opposing surfaces defining the fluid conduit there between, and a plasma generation system (102) which is operatively coupled to the electrodes (56, 58) for supplying power to the electrodes to form or maintain a plasma from etchant gases within the fluid conduit, characterized in that the fluid conduit (54) includes two mirror image left and right path portions, and in that one of the electrodes (56) includes a center wall member (57) that divides gas flow through the fluid conduit into the left and right path portions. An apparatus as claimed in claim 1, wherein the fluid conduit (54) formed between the opposing surfaces defines a plurality of collection chambers (62) formed sequentially within the fluid conduit between the inlet and the outlet. An apparatus as claimed in claim 2, wherein said opposing electrode surfaces define a labyrinthol/serpentine fluid conduit. An apparatus as claimed in claim 1, wherein said center wall member (57) includes an upper edge having an angle of 30 degrees or less. Apparatus according to any of the preceding claims, characterized in that the gas passage way (54) is enclosed by a removable door (63) along with a backplate (65). Apparatus according to anyone of the preceding claims, characterized in that the electrodes (56, 58) comprise protruding fingers (79) for directing said gas flow through each U-turn. Apparatus to be connected in an exhaust line of a substrate processing chamber for minimizing deposition in an exhaust line of the substrate processing chamber comprising gas passageway (435) having an inlet (434), an outlet (436) and a collection chamber between the inlet and the outlet, the collection chamber being formed by the gas passageway (435) including at least one U- shaped portion for mechanically collecting particulate matter flowing through the fluid conduit and to inhibit egress of the particulate matter from the collection chamber, first and second electrodes (430, 432) each having opposing surfaces defining the gas passageway therebetween, and a plasma generation system which is operatively coupled to the electrodes for supplying power to the electrodes to form or maintain a plasma from etchant gases within the gas passageway, characterized in that the first and second electrodes (430, 432) are evenly spaced, parallel electrodes, and that the gas passageway (435) includes two gas flow paths (435A, 435B) that are positioned side by side and that are initially separated by a portion of electrode (430). Apparatus of any of the preceding claims, further comprising an electrostatic particle trapping system (100; 420) operatively coupled to the electrodes (56, 58; 402, 404; 430, 432) for applying a voltage between the electrodes to collect particle matter on the opposing electrode surfaces. Method of for minimizing deposition in an exhaust line of a substrate processing chamber comprising: connecting an apparatus according to any of the preceding claims in an exhaust line of a substrate processing chamber, and introducing etchant gases exhausted through the exhaust line from the substrate processing chamber during a cleaning operation of the substrate processing chamber. The method of claim 9, wherein etchant gases are introduced into the exhaust line upstream of the apparatus and downstream from the substrate processing chamber. The method of claim 9, wherein etchant gases are introduced directly into the fluid conduit.

Inventors: CHEUNG DAVID, FODOR MARK, PANG BEN, RAOUX SEBASTIEN, TAYLOR WILLIAM N JR, CHEUNG, DAVID, FODOR, MARK, PANG, BEN, RAOUX, SEBASTIEN, TAYLOR, WILLIAM N., JR.
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1997
Main classification: C23C16
IPC codes: B01D53, B01D45, B03C3, H01L21, C23C16