Method and apparatus for minimizing deposition in an exhaust line

Abstract: An apparatus for minimizing deposition in an exhaust line of a substrate processing chamber. The apparatus includes first and second members having opposing surfaces that define a fluid conduit between them. The fluid conduit includes an inlet, an outlet and a collection chamber between the inlet and the outlet. The apparatus is connected at its inlet to receive the exhaust of the substrate processing chamber, and 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 microwave plasma generation system (450) supplies microwave energy within the fluid conduit 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 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 effecting 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 effects 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. One approach that has been employed to clean the foreline relies on a scrubbing system that uses plasma enhanced CVD techniques to extract reactive components in the exhaust gas as film deposits on electrode surfaces. The scrubbing system is designed to maximize the removal of reactants as a solid film and uses large surface area spiral electrodes. The spiral electrodes are contained within a removable canister that is positioned near the end of the foreline between the blower pump and mechanical pump. After a sufficient amount of solid waste has built up on the electrodes, the canisters may be removed for disposal and replacement. Problems exist in this prior art method in that the system relies on the large surface area of the electrodes to provide an area for deposited solid matter to collect. To accommodate the large surface area of the electrodes, the system is necessarily large and bulky. Furthermore, extra expenses are incurred in the operation of this prior art scrubber system since the removable canister is a disposable product that must be replaced and properly disposed. Also, the scrubbing system is located downstream from a beginning portion of the vacuum foreline and thus does not ensure removal of powdery material or particulate matter that builds-up in this portion of the line. From the above it can be seen that an improved method and apparatus for cleaning a foreline is desirable. Another issue of concern in CVD and other substrate processing apparatus relates to the types of gases and byproducts exhausted from the processing chamber through the foreline. For example, because dissociation of gas within the cleaning plasma is not complete (in some applications only 10% of the introduced gas molecules are dissociated), and the residence time in the chamber of individual molecules in the cleaning gas is relatively short, only a small portion of the molecules released into the chamber react with the deposited material. The gas molecules that do not take part in an etchant reaction are pumped out of the chamber along with the etched away material and reaction byproducts through a vacuum line that is commonly referred to as the "foreline." The exhausted gases are emission byproducts of the semiconductor process. The EP 0 289 858 shows a particle collector trap for a vacuum evacuating system wherein a vessel formed of a double-wall cylinder is provided which vessel has an inlet conduit connected to a vacuum processing chamber and an outlet conduit connected to at least one vacuum pump. A passage for flowing gas is defined between the double walls of the vessel. One of the double walls is heated and the other wall is cooled so at to maintain a predetermined temperature difference is therebetween. The CA-A-1 282 732 shows an apparatus for the treatment of post-reaction gases wherein residual products are caused to be deposited out of the gases, before reaching the pump, at the least greatly reducing the percentage of residual products in the gases. The gas is fed through a radio frequency plasma, the residual products being caused to deposit on one or more interior surfaces of the post-reaction chamber within which the plasma is formed. The EP 0 781 599 which was filed naming the present inventors with a publication date of July 2, 1997 and priority dates of December 27, 1995 and October 30, 1996 shows a method and an apparatus for reducing perfluorocompound gases from substrate processing equipment emissions. With the priority of December 27, 1995, there is disclosed an apparatus comprising a gas passage way arranged between an inlet and on outlet and defined by electrodes with opposing electrodes surfaces, one of the electrodes being connected to a DC power supply. A microwave plasma generation system is coupled to a wave guide to generate a plasma within the gas passage way. It is the object of the invention to provide an effective apparatus and a method 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 an apparatus as claimed in claim 1 and a method as claimed in claim 7. Advantageous embodiments of the invention are given in the sub-claims. 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. Also, in some preferred embodiments, the present invention does not require the use of additional gases and/or consumable parts. 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. I; Fig. 3 illustrates a second method of connecting the present invention to the chemical vapor deposition apparatus of Fig. 1; Fig. 4 is an electrical diagram of the circuit connected to and including electrodes; Fig. 5 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. 6 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. 7(a) is a side cross-sectional view of an embodiment of the vacuum line cleaning apparatus of the present invention that uses microwave power to form a plasma; Fig. 7(b) is a front elevational view of the vacuum line cleaning apparatus of Fig. 7(a); Figs. 8(a), 8(b) and 8(c) are graphs showing the voltage wavefonns generated by the microwave power supply in the apparatus of Fig. 7(a); Fig. 9(a) is a side cross sectional view of a second embodiment of the vacuum line cleaning apparatus of the present invention that uses microwave power to form a plasma; and Fig. 9(b) is a front elevational view of the vacuum line cleaning apparatus of Fig. 9(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. I 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 12 in a vacuum chamber 15. Susceptor 12 is highly thermally responsive. Susceptor 12 (and the wafer supported on the upper surface of susceptor 12) can be controllably moved between a lower loading/off-loading position and an upper processing position 14 which is closely adjacent manifold 11. When susceptor 12 and the wafer are in processing position 14, 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 12 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 12. 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 12 between a processing position 14 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 temperatuze, 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 be 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 tile 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. Fig. 4 is a diagram showing the electrical circuit that includes electrodes 56 and 58. As shown in Fig. 4, 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 (about 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. 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. 5, 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. 5, 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. 6 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 122 shows the gravitational force, line 124 shows the electrostatic force and line 126 shows the particle neutral drag force. As shown, for smaller particles, electrostatic force 124 is greater than the gravitational force 122. For larger particles, gravitational force 122 is dominant over electrostatic force 124. 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. 6 is that DPA 40 is preferably designed so at least one of electrostatic force 124 or gravitational force 122 should be larger than the neutral drag force 126 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 Joules 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 128. While thermophoretic force 128 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 the create larger temperature gradients such that a larger thermophoretic force is created 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. 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. Fig. 7(a) is a side cross-sectional view of another embodiment of DPA 40, and Fig. 7(b) is a front elevational view of the embodiment shown in Fig. 7(a). The embodiment of Figs. 7(a) and (b) employs microwave sources to generate a plasma and remove particulate material and residue collected within the DPA. While a number of different microwave sources are available, for purchase and operational cost reasons, a pair of alternatively pulsed magnetrons 450 (e.g., the type of magnetron employed in some microwave ovens) is preferably used. Such magnetrons can be two orders of magnitude or more lower than the price of a CW microwave generator or an RF generator. As shown in Fig. 8(a), each magnetron 450 generates an alternatively pulsed (60 Hz) electric field (2.45 GHz). By delaying the pulse of one magnetron by a phase difference of 180 degrees in relation to the other magnetron, as shown in Fig. 8(b), the two magnetron sources can be constantly pulsed at 120 Hz as shown in Fig. 8(c). In Fig. 8(c), a first cycle of the waveform (M1) is generated by one of the magnetrons and a second cycle (M2) is generated by the other. The energy generated by the magnetrons can produce an ionization efficiency of close to 90% with a high plasma density. Thus, such a power source results in a higher clean efficiency than capacitively coupled electrodes, which typically produce an ionization efficiency of between 10-20%. A further advantage of the microwave sources is the reduction of Joules effect heating. Because less heat is generated, electrodes 452 and 454 (Fig. 7(a)), which define a gas flow passageway 456, can be readily made out of relatively thin sheet metal. Gas flow passageway 456 starts at an inlet 458 and ends at an outlet 460. The passageway is divided and follows dual paths in a manner similar to gas passageway 435 in the embodiment of Fig. 16(a). Also, inlet 458 is flared out into DPA 40 at the start of gas passageway 456 as shown by profile 464 in Fig. 7(b). Magnetrons 450 are positioned on opposing sides of DPA 40. The microwave power is coupled to the reactor by an appropriate waveguide 462 (Fig. 7(b)). The magnetrons and waveguide combine to project microwaves through the width of gas passageway 456 so that plasma formation can occur throughout the entire gas passageway. The distance between the sheet metal electrodes may be adjusted according to the wavelength of the microwave so that each node (point of zero intensity) of the electric field is located at the electrode surface (i.e., the distance between the electrode plates should be a multiple of one-half the period of the microwave wavelength). Because of the positioning of magnetrons 450 and waveguide 462, a plasma is formed in all parts of gas passageway 456. A ceramic door 466 (Fig. 7(b)) separates the magnetrons and waveguide from electrodes 452 and 454, and an outer casing 468 encloses the DPA and provides a second level seal. A DC power source (not shown) is coupled to electrode 452 to provide an electrostatic collector as previously described during a deposition or other substrate processing operation. DC power to electrode 452 is switched OFF (by a switch not shown) and the electrode is grounded, however, when magnetrons 450 are activated during a clean operation. Switching electrode 452 to ground during this time is required to prevent arcing, which might otherwise occur. Fig. 9(a) is a side cross-sectional view of another embodiment of DPA 40, and Fig. 9(b) is a front elevational view of the embodiment shown in Fig. 9(a). The embodiment of Figs. 9(a) and (b) is similar to the embodiment of Figs. 7(a) and (b) in that it employs magnetrons 450 to generate the DPA plasma. As shown in Fig. 9(a), this embodiment of DPA 40, however, the DPA includes an initial module 472 located just downstream of an inlet 474. Module 472 is dedicated toward plasma generation during a clean cycle so that etchant radicals (e.g., CF x and free F when CF 4 is employed as an etchant gas) can be generated with increased ionization efficiency. Such generated radicals have a relatively long lifetime and remain active when they are pumped into a second module 475 of the DPA to react with deposited and collected material. Second module 475 includes gas passageway 470 that is defined by opposing electrodes 476 and 480 constructed from sheet metal in a preferred embodiment. Gas passageway 470 is similar in design to gas passageway 456 in the embodiment of Figs. 17(a) and (b). It includes dual passages 470a and 470b and ends at outlet 478. A waveguide 482 is coupled to magnetrons 450. The waveguide and magnetrons are positioned so that microwave generation forms the plasma in module 472. An inner wall of anode 476 prevents the microwaves from reaching other portions of gas passageway 470 outside of module 472. Electrode 480 is connected to a DC power supply (not shown) to provide an electrostatic collector similar to those previously described. In this embodiment, DC power to electrode 480 does not need to be switched OFF during the clean cycles. Because no plasma is generated in second module 475, there is no arcing problem.

Claims: Apparatus for minimizing deposition in an exhaust line of a substrate processing chamber comprising a gas passage way (456, 470) arranged between an inlet (458, 474) and an outlet (460, 478) and defined by electrodes (452, 454, 476, 480) with opposing electrode surfaces, one of the electrodes (452, 480) being connected to a DC power supply, and a microwave plasma generation system coupled to a waveguide (462, 482) to generate a plasma within said gas passage way (456, 470), characterized in that said microwave plasma generation system comprises a pair of magnetrons (450) adapted to be alternatively pulsed and that in use of the apparatus the pulse of one magnetron (450) is delayed by a phase difference of 180 degrees in relation to the other magnetron (450). Apparatus according to claim 1, characterized in that the magnetrons (450) are positioned on opposing sides of said apparatus. Apparatus according to claim 1 or 2, characterized by comprising a switch to switch off DC power to said one electrode (452, 480) during a substrate clean operation. Apparatus according to anyone of the claims 1 to 3, characterized by comprising an initial module (472) located downstream of the inlet (474) and a second module (475) including the gas passage way (470), the waveguide (482) and the magnetrons (450) being positioned such that the microwaves are prevented from reaching other portions of gas passage way (470) outside of the initial module (472). Apparatus according to anyone of the claims 1 to 4, characterized in that the gas passage way (456, 470) includes at least one U-turn for mechanically collecting particular matter flowing through the apparatus. Apparatus according to anyone of the claims 1 to 5, characterized in that the gas passage way (456, 470) includes two gas flow paths, which are positioned side by side and which are initially separated by a portion of one electrode (452, 480). A method for minimizing deposition in an exhaust line of a substrate processing chamber, wherein exhaust gases from the substrate processing chamber are pumped through a gas passage way (456, 470) defined by electrodes (452, 454, 476, 480) with opposing electrode surfaces, wherein one of the electrodes (452, 480) is connected to a DC power supply and a plasma generated within said gas passage way by a microwave plasma generation system coupled to a waveguide (462, 482) to (456, 470) characterized in that said plasma is generated by a pair of alternatively pulsed magnetrons (450), the pulse of one magnetron (450) being delayed by a phase difference of 180 degrees in relation to the other magnetron (450). Method according to claim 7, characterized in that DC power to said one electrode (452) is switched off during a substrate clean operation.

Inventors: CHEUNG DAVID, FAIRBAIRN KEVIN, KELKAR MUKUL, PONNEKANTI HARI, RAOUX SEBASTIAN, TANAKA TSUTOMU, CHEUNG, DAVID, FAIRBAIRN, KEVIN, KELKAR, MUKUL, PONNEKANTI, HARI, RAOUX, SEBASTIAN, TANAKA, TSUTOMU
Applicants: APPLIED MATERIALS INC, APPLIED MATERIALS, INC.
Application year: 1997
Main classification: C23C16
IPC codes: C23F4, B01J19, C23C16, H01J37, H01L21, H05H1