Method for sub-atmospheric gas delivery with backflow control

[0002] This application is a Divisional of U.S. application Ser. No. 09/307,650, now U.S. Pat. No. 6,155,289, filed May 7, 1999.

[0003] The present invention relates to a method of gas delivery from a sub-atmospheric gas source, and in particular to a backflow control method suitable for same, that prevents gas from backflowing into the sub-atmospheric gas source.

[0004] Conventional high-pressure gases are typically delivered to process tools within semiconductor fabricators (e.g., chemical vapor deposition or CVD tools) by complex gas distribution systems. Flammable, pyrophoric, and toxic compressed gases such as silane, phosphine (PH₃), and arsine (AsH₃) (pure or mixed) typically require a multiplicity of equipment for safe storage and delivery. Such equipment typically includes a gas cabinet, one or more valve manifold boxes (“VMBs”), and gas isolation boxes (“GIBs”). The gas cabinet is typically positioned remotely from the process tool or the semiconductor fabricator itself. VMBs and GIBs are typically located locally within the fabricator. These systems are typically connected with gas piping or tubing and the functions of each sub-system, when coupled together, provide total gas management (e.g., purge, evacuation and process gas supply) for a particular flammable, pyrophoric, or toxic compressed gas.

[0005] At the tool level, because CVD processes run at medium vacuum levels (10⁻¹-10⁻⁴torr), controlling the dopant partial pressure and overall pressure in the process chamber of the process tool becomes increasingly difficult when using dilute gas mixtures of such gases. The inclusion of balance gases such as helium or argon contributes additional gas molecules in the process chamber of the process tool, which results in higher operating pressures and reduced film deposition rates. Specifically, dilute fixed phosphine mixtures do not provide adequate dopant incorporation in a PSG film unless the flow of the PH₃mixture is increased significantly. However, increasing the flow of the dilute PH₃mixture has the effect of lowering the film deposition rate and negatively altering the process capability. In addition, there is the safety risk of a catastrophic release of gas with a high-pressure gas delivery system.

[0006] The use of sub-atmospheric gas (i.e., gas that is below 1 atmosphere) as a source gas is an alternative to high pressure gas delivery systems. Such a gas source virtually eliminates the risk of catastrophic gas releases, thus allowing the safe storage and use of such gases at 100% concentration.

[0007] Sub-atmospheric gas sources are typically stored locally in the fabricator close to the process tools for which they are intended. This minimizes pressure losses across the delivery path and, therefore, increases the deliverable quantity from the sub-atmospheric gas cylinder. Conventional gas cabinets configured for compressed gases typically do not provide adequate safety and are not capable of sub-atmospheric gas delivery or management. Gas cabinets configured for compressed gases provide neither the ability to pump purge the entire delivery path effectively, nor the ability to extract gases from the process tool to the cabinet itself. Additionally, conventional gas cabinets configured for compressed gases do not generally provide suitable capability to sense, prevent and mitigate the backflow of gases to the sub-atmospheric gas cylinder.

[0008] Lastly, conventional compressed gas cabinets do not provide sufficient detection of in-board leaks to the system and gas cylinder, mainly due to the fact the conventional systems for compressed gases are meant to operate at greater than 0 psig. Also, the limitations of conventional gas cabinets can not be overcome for sub-atmospheric gases by merely providing the additional functionality of equipment such as VMBs and GIBs. Providing the external functionality of VMBs and GIBs is not a practical option due to installation space restrictions, pressure drop restrictions, and, potentially, gas flow restrictions.

[0009] The present invention relates to a method of gas delivery from a sub-atmospheric gas source, and in particular to a backflow control method suitable for same, that prevents gas from backflowing into the sub-atmospheric gas source.

[0010] Accordingly, the invention includes a method of preventing backflow into a source of sub-atmospheric gas, using the apparatus as described immediately above. The method comprises the steps of first, measuring a first pressure in the gas line between the sub-atmospheric gas source and the flow restrictor, then measuring a second pressure in the gas line between the flow restrictor and the chamber, then comparing the first pressure to the second pressure, and then closing the valve when the second pressure exceeds the first pressure.

[0011] FIG. 1 is a schematic diagram illustrating a backflow control apparatus usable with the method of the present invention; and

[0012] FIG. 2 is a schematic diagram illustrating a gas delivery system usable with the method of the present invention which includes a backflow control apparatus in accordance the present invention.

[0013] The present invention relates to a method of and system for gas delivery from a sub-atmospheric gas source, and in particular to a backflow control apparatus and method suitable for same, that prevents gas from backflowing into the sub-atmospheric gas source. The delivery system described herein provides for the complete gas management of sub-atmospheric gases for a variety of applications, including semiconductor manufacturing and process equipment therefor. The present invention is an extension of U.S. Pat. No. 5,518,528, which patent is herein incorporated by reference. First, the backflow control apparatus is described in the context of a simplified gas delivery system. Then a complete sub-atmospheric gas delivery system incorporating the backflow apparatus of the present invention is set forth.

[0014] With reference to FIG. 1, a backflow control apparatus 10 for preventing gas backflow is now described in the context of its use in a simplified gas delivery system 12. The latter comprises a sub-atmospheric gas source 14 for providing a first gas. Sub-atmospheric gas source 14 may be, for example, a cannister containing PH₃gas adsorbed in a microporous media at a pressure less than 760 torr, e.g., between 600 torr and 20 torr at room temperature (21° C.).

[0015] System 12 further includes a valve 20 attached to or in immediate fluid communication with gas source 14. A gas line 26 is connected to valve 20. The latter is a blocking valve capable of blocking fluid communication between sub-atmospheric gas source 14 and gas line 26 upon receipt of a signal (e.g., a pneumatic signal or an electrical signal). Gas line 26 may be, for example, seamless stainless steel tubing with a maximum internal finish of 10 Ra. Along gas line 26 is arranged, in order from valve 20, a first pressure transducer P1, a flow restrictor R, and a second pressure transducer P2. First and second pressure transducers P1 and P2 detect the pressure of gas in gas line 26, and provide electrical output signals that vary as a function of changes in pressure in the gas line. Transducers P1 and P2 are preferably rated at 0-50 psia with a 4-20 ma output and a sensitivity of 1%. Flow restrictor R restricts the flow of gas in gas line 26, and may be any one of those known in the art, such as a mass flow controller or a filter. An exemplary flow restrictor R has a 10 Ra internal finish and is rated for 25 LPM at 30 psig nitrogen.

[0016] Gas delivery system 12 further includes a valve controller unit 40 electronically connected with first and second pressure transducers P1 and P2 (or, more generally, pressure difference sensors) and valve 20. Valve controller unit 40 may be, for example, a programmable logic controller (PLC) capable of receiving and processing signals from first and second pressure transducers P1 and P2, and generating a control signal, as described in more detail below.

[0017] Gas delivery system 12 also includes a chamber 50 at which gas line 26 terminates. Chamber 50 may be, for example, a chemical vapor deposition (CVD) process chamber wherein thin films are epitaxially deposited on silicon wafers as part of the process for manufacturing semiconductor devices. Connected to chamber 50 is a gas source 52, which provides one or more additional gases (e.g., O₂) to chamber 50 for carrying out reactions therein. Also connected to chamber 50 is a vacuum pump 54 for maintaining the chamber at a suitably low pressure.

[0018] With continuing reference to FIG. 1, backflow control apparatus 10 of the present invention comprises valve 20, gas line 26, first and second pressure transducers P1 and P2, flow restrictor R, and valve controller unit 40 capable of providing the aforementioned signal to valve 20.

[0019] The operation of backflow control apparatus 10 as part of gas delivery system 12 is now described. When gas delivery system 12 is functioning properly, the pressure in chamber 50 is less than that of sub-atmospheric gas source 14 so that gas can flow from the sub-atmospheric gas source to the chamber. However, it is possible that the pressure in chamber 50 can become greater than that in sub-atmospheric gas source 14, resulting in the flow of gas from the chamber to the sub-atmospheric gas source. This phenomenon is referred to herein as “backflow.” A backflow situation may arise, for example, by the failure of vacuum pump 54, or by an excess flow of one or more gases from gas source 52. Backflow may also occur due to a leak in system 12 (e.g., in gas line 26). Backflow of a second gas (i.e., a gas other than the gas present in sub-atmospheric gas source 14) into sub-atmospheric gas source 14 is particularly undesirable for two main reasons. The first reason is that the second gas will contaminate sub-atmospheric gas source 14. The second reason is that certain combinations of gases can result in dangerous reactions, creating a safety hazard. For example, the combination of PH₃gas and O₂gas (from the atmosphere or from gas source 52) can result in a dangerously exothermic reaction.

[0020] Accordingly, with continuing reference to FIG. 1, during operation of gas delivery system 12, backflow control apparatus 10 operates to prevent backflow as follows. Pressure transducer P1 monitors the pressure p1 in gas line 26 between valve 20 and flow restrictor R, while pressure transducer P2 monitors the pressure p2 in the gas line between the flow restrictor and chamber 50. Under normal operating conditions, pressure p1 is greater than pressure p2. For example, in a gas delivery system for a CVD process tool, p1 may range from 20-600 torr (e.g., 600 torr for a full cylinder and 20 torr for a nearly empty cylinder), while p2 may range from 0-20 torr, with P1>P2.

[0021] During the operation of gas delivery system 12, pressure transducers P1 and P2 generate first and second signals, respectively, corresponding to pressures p1 and p2, respectively. These first and second signals are sent to valve controller unit 40, which performs a comparison of the signals. If p1 becomes less than p2, the pressure gradient has reversed, allowing backflow of gas into sub-atmospheric gas source 14. Accordingly, when pressure p1 is less than p2, or alternatively, when pressure p1 is within a certain value (set point) to pressure p2 (e.g., within 5 torr), valve controller unit 40 sends the aforementioned signal (e.g., a third signal) to valve 20. This third signal causes valve 20 to close, thereby preventing backflow of gas into sub-atmospheric gas source 14.

[0022] With continuing reference to FIG. 1, as an optional addition, backflow control apparatus 10 may include a temperature sensor 60 in thermal communication with sub-atmospheric gas source 14 and in electronic communication with valve controller unit 40. Temperature sensor 60 may be, for example, a thermocouple attached to sub-atmospheric gas source 14 or an infra-red detector for sensing infra-red radiation from the sub-atmospheric gas source. Temperature sensor 60 monitors the temperature of sub-atmospheric gas source 14 by sending a signal corresponding to the temperature to valve controller unit 40. If there is backflow from chamber 50, or if there is a leak in gas line 26 such that an exothermic reaction occurs in sub-atmospheric gas source 14, the temperature of the gas source will rise. The rise in temperature of sub-atmospheric gas source 14 is then sensed by valve controller unit 40, which sends a signal to valve 20 causing it to close. At this point, valve controller unit 40 may also display a visual signal or emit an audio signal when the temperature rises to a threshold temperature or follows a predetermined temperature vs. time curve, stored within the backflow controller, characteristic of an exothermic reaction with the gas source. Such temperature vs. time curves can be determined empirically.

[0023] Alternatively, temperature sensor 60 may be a mechanical temperature thermo-switch hardwired directly to valve 20. The thermo-switch detects the temperature of sub-atmospheric gas source 14. When the gas source temperature reaches a predetermined value, the thermo-switch sends a signal directly to valve 20 to close the valve, rather than via valve controller unit 40.

[0024] With reference now to FIG. 2, a complete sub-atmospheric gas delivery system 100 incorporating backflow control apparatus 10 of FIG. 1 is discussed. Where appropriate, the same reference symbols as used in gas delivery system 10 are used to describe gas delivery system 100.

[0025] Gas delivery system 100 comprises a sub-atmospheric gas source 14, valve 20, a distribution manifold 108, and valve control unit 40. Gas delivery system 100 is connected to chamber 50, which is in fluid communication with second gas source 52 and vacuum pump 54. Gas delivery system 100 also preferably includes a cabinet 114 (dashed line) enclosing a portion of the gas delivery system.

[0026] Distribution manifold 108 comprises three different interconnected branches or “sticks”: a purge stick 120, a process gas delivery stick 124, and an evacuation (“venturi”) stick 130. Purge stick 120 includes purge gas source 140, such as a cannister of nitrogen (N₂) or other inert gas, and a purge gas line 144. Along the latter is included, in order from purge gas source 140, a pressure switch PS1, a first restrictor R1, a first filter F1 and an first actuated valve AV1. Process gas delivery stick 124 includes sub-atmospheric gas source 14, valve 20 which is controlled by valve controller unit 40, and a gas line 150 connected to valve 20. Process gas delivery stick 124 further includes, in order along gas line 150 from valve 20, a first pressure transducer PT1, first actuated valve AV1 (in common with purge stick 120), a second actuated valve AV2, one or more second isolation valves IV1, and a second pressure transducer PT2 arranged directly downstream from the first isolation valve IV1 that is farthest from gas source 14 and still within cabinet 114. Process gas delivery stick 124 also preferably includes along gas line 150, between cabinet 114 and chamber 50, one or more additional isolation valves, such as valves IV2-IV4, one or more filters, such as second filter F2, and one or more additional flow restrictors, such as second flow restrictor R2. Process gas delivery stick 124 also includes a third pressure transducer PT3 arranged along gas line 150 chamber-wise (i.e., downstream) of flow restrictor R2 and in electronic communication with valve controller unit 40.

[0027] With continuing reference to FIG. 2, evacuation stick 130 comprises an evacuation gas source 154, such as an N2 gas cannister, and an evacuation gas line 160 connected thereto. Evacuation stick 130 further comprises, in order along gas line 160 from gas source 154, a fifth isolation valve IV5 and a vacuum generator VT, Such as a venturi type vacuum generator. Stemming from vacuum generator VT is a branch evacuation gas line 164 leading to second actuated valve AV2, thereby connecting evacuation stick 130 and process gas delivery stick 124. A vacuum switch VS1 is arranged within line 164 between vacuum generator VT and valve AV2. Evacuation line 160 continues from vacuum generator VT and leads to a vent 170.

[0028] The operation of gas delivery system 100 is now described. First, purge stick 120 is used to purge process gas delivery stick 124 of reactive gases. Accordingly, purge stick 120 provides, from purge gas source 140, compressed (e.g., 5-30 psig) inert gas, such as N2. The purge gas is introduced into process gas delivery stick 124 by actuating actuated valve AV1 so that the purge gas can flow into process gas line 150 toward chamber 50. After the purge step is complete, actuated valve AV1 is closed. After the purge process, evacuation of process gas delivery stick 124 is accomplished by activating evacuation stick 130. This involves closing one of isolation valves IV2-IV4, and adjusting actuated valve AV2 so that branch evacuation line 164 is in fluid communication with gas line 150. Isolation valve IV5 is then opened and vacuum switch VS1 is activated to allow vacuum generator VT to communicate with gas line 150. In this arrangement, the flow of compressed (e.g., 60 psi) gas through evacuation line 160 and vacuum generator VT creates negative pressure between gas line 150 and evacuation line 160, drawing gas present in process gas delivery stick 124 through evacuation line 164, to evacuation line 160 and out of the system through vent 170.

[0029] After process gas delivery stick 124 is evacuated, the next step is to deliver the process gas to chamber 50 using the process gas delivery stick. This is accomplished by opening cylinder valve 20 and adjusting valves IV1-IV4 so that gas from sub-atmospheric gas source 14 can flow into chamber 50. Pressure transducer PT2, located adjacent and downstream of first isolation valve IV2, is used primarily to monitor the pressure downstream of the first isolation valve when this valve is in the closed position. As discussed above in connection with gas delivery system 10, the pressure within chamber 50 is maintained at a value less than that of sub-atmospheric gas source 14, resulting in a pressure gradient that causes gas within sub-atmospheric gas source 14 to flow toward chamber 50.

[0030] As mentioned above, gas delivery system 100 includes backflow control apparatus 12, namely, in the present arrangement, valve 20, gas line 150, first and third pressure transducers PT1 and PT3 (serving the function of pressure transducers P1 and P2 of FIG. 1), flow restrictor R2, valve controller unit 40, and preferably temperature sensor 60 in electronic communication with valve controller unit 40, or alternatively, in direct electronic communication with valve 20. As described in detail above in connection with gas delivery system 10, backflow control apparatus 12 prevents the backflow of gas to sub-atmospheric gas source 14 and, in a preferred embodiment, is also capable of monitoring a temperature rise in the sub-atmospheric gas source and closing off same.

[0031] While the present invention has been described in connection with preferred embodiments, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.