Automatic reference-pressure balance method

The present invention provides a system and method for calibrating pressure sensors associated with chambers in a processing facility. The system calibrates the pressure sensors while the chamber are open to each other, such as through an open slit valve or vacuum sealed door. Maintaining the pressure in the chambers relative to each other prevents a rush of gases, condensate or other foreign materials into an adjacent chamber that may occur when the pressure between the chambers is not equalized. This prevents contamination of the materials being processed, and eliminates the need for system shut-down to calibrate sensors. Also, since calibration occurs every time the slit valve is open, the calibration is real-time and does not allow the pressure differential between the chambers to become too great.

TECHNICAL FIELD
 This invention relates generally to semiconductor processing and, more
 particularly, to a system and method for calibrating gas or liquid
 pressure during wafer processing.
 BACKGROUND
 Contamination of materials is a factor in many manufacturing processes, and
 is of particular concern in the fabrication of integrated circuits. In
 general, integrated circuit technology is based on the ability to form
 numerous transistor structures on a single semiconductor substrate.
 Typically, multiple integrated circuits will be formed on a single silicon
 wafer, the wafer providing the semiconductor substrate for the circuits.
 The intricacy of the circuits and the large number of steps involved in
 the fabrication make it essential that each of the process steps be
 tightly controlled and meet very stringent specifications to prevent any
 type of contamination. To increase the purity and hence the quality and
 reliability of manufacturing processes, most are done in sealed rooms or
 chambers, where the environment, including temperature, pressure and
 purity of liquids or gases introduced can be controlled. One of the
 biggest sources of contamination occurs when the product being
 manufactured is transferred from one area of the manufacturing process to
 the next, which necessitates opening a door or valve to introduce the
 material into the next chamber or room. When the door or valve is open, if
 there is a pressure differential between the chambers, the potential for
 contamination is increased. Processing fluid and/or gas will rush from the
 chamber with the higher pressure to the chamber with the lower pressure to
 equalize the pressure, bringing solids and liquids along with the gas,
 which may cause such problems as condensation and particulate
 contamination on the material being manufactured.
 The solutions used to mitigate the above-identified problem have included:
 reducing the amount of time that the door or valve is opened to reduce the
 amount of contaminants that enter the chamber; making the transfer in two
 stages; utilizing an intermediate or transfer chamber into which only one
 of the other chambers is opened at a given time; placing calibrated
 pressure sensors in each chamber; and tying the pressure sensors into the
 valve-opening mechanisms in both chambers to obtain a desired, consistent
 pressure balance before the valve between the chambers is opened. However,
 these solutions, separately or in combination, do not always adequately
 resolve the aforementioned problems.
 Even minimizing the time the door or valve is open will result in some
 contamination, especially if the pressure is not equalized between the two
 chambers before opening the door or valve. One of the biggest problems in
 equalizing the pressure is keeping the pressure sensors calibrated, as
 sensors tend to drift in calibration over time. If the pressure sensor in
 either chamber is out of calibration, the pressure between the two
 chambers will not actually be equal, and when the door or valve is open,
 the gas will rush from the higher to lower pressure chamber to equalize
 the pressure. The effective drift of the pressure sensors is actually
 doubled if the two sensors drift calibration in opposite directions. If a
 sensor has drifted, process overseers are generally unaware of the problem
 until a rush of gas between the chambers has occurred, resulting in
 contamination of a manufactured product. In order to re-calibrate sensors
 that have drifted, the manufacturing process generally has to be shut
 down, and the sensor taken off-line to be calibrated, resulting in
 production down-time.
 U.S. Pat. No. 5,808,175 issued Sep. 15, 1998 to Shen-Yan Chang discloses a
 method of temporarily, manually mounting a second, in-line calibrated
 sensor to the same chamber for the purpose of monitoring or correcting the
 first sensor. However, Chang only utilizes the second sensor for
 comparison to the readings obtained from the first sensor for the same
 chamber, to determine if it needs replacing. If there is drift in the
 sensors used to read the pressure in different chambers, a situation may
 still occur wherein the pressure differential between two chambers is such
 that a rush of gas and contaminants occurs when the door or valve between
 the two chambers is opened.
 It would, therefore, be desirable to be able to provide a method and
 apparatus wherein the pressure in the two chambers between which materials
 are being transferred can be kept equal so that there will not be a rush
 of gas between the two chambers when the door or valve is opened.
 SUMMARY
 The present invention overcomes the above outlined problems and a technical
 advance is achieved by a system and method that equalizes and calibrates
 the pressure of two or more chambers on either side of a valve (door)
 during operation. In one embodiment, the method is performed each time the
 valve is opened. When the valve is opened, the pressure in the chambers
 will equalize. After the pressure in the chambers has equalized, pressure
 readings from sensors mounted in each chamber are calibrated relative to
 each other.
 In some embodiments, the pressure readings are sent to a control module The
 control module evaluates the readings taken from the sensors and adjusts
 them to match each other.
 In some embodiments, there may be an intermediate, or transfer chamber
 between a process chamber and a loading chamber. The sensor readings from
 the process and loading chambers are adjusted to match the sensor readings
 of the transfer chamber. As a result, all the chambers will be calibrated
 with the transfer chamber, and therefore with each other.
 Since calibration occurs every time the valve is open, the pressure
 differential between the chambers never becomes too great. As a result,
 there is little if any fluid flow (e.g., a flow processing gases and/or
 contaminants) between the chambers and a very clean chamber environment is
 maintained.
 Also, because the relative calibration is done during actual use, the
 system does not have to be shut down to perform calibration routines.
 This, of course, results in increased productivity and cycle time.

DETAILED DESCRIPTION
 Referring to FIG. 1, reference numeral 10 designates a portion of an
 integrated circuit processing facility including two loading chambers 12,
 14, a transfer chamber 16, and two process chambers 18, 20. Loading
 chambers are typically used for loading and unloading one or more
 materials for processing. Processing chambers are typically used for
 performing processing operations, and may include additional components
 for varying the temperature of the chamber or for adding processing fluids
 and/or constituents into the chamber. Transfer chambers are often used as
 an intermediate chamber between loading chambers and processing chambers,
 and may include a robot arm or some equivalent device for moving materials
 between the different chambers. To facilitate processing and maintain a
 clean environment, the different chambers must be hermetically sealed from
 each other at particular times.
 The loading chambers 12, 14 are connected to the transfer chamber 16
 through slit valves 22, 24, respectively. Likewise, the process chambers
 18, 20 are connected to the transfer chamber 16 through slit valves 26,
 28, respectively. Connected to the transfer chamber 16 is an exhaust line
 40 with a variable restriction control valve 42 used for controlling the
 pressure in the chamber. Likewise, connected to the loading and process
 chambers 12, 14, 18, 20 are exhaust lines 44, 46, 48, 50 with variable
 restriction control valves 52, 54, 56, 58, respectively. Each of the
 variable restriction control valves 42, 52, 54, 56, 58 is controlled by a
 control module 60.
 The control module 60 receives pressure readings from pressure sensors
 (e.g. transducers) 62, 64, 66, 68, 70 connected to loading chambers 12 and
 14, transfer chamber 16, and process chambers 18, 20, respectively.
 Furthermore, the control module 60 is informed of the status (open or
 closed) of slit valves 22, 24, 26, and 28 through monitor lines 72, 74,
 76, 78 respectively.
 In operation, the control module 60 reads the pressure of the loading
 chamber 12 through the pressure sensor 64 and the transfer chamber 16
 through the pressure sensor 62. The control module 60 then equalizes the
 pressure in the two chambers 16 and 12 by adjusting the variable
 restriction control valves 42 and 52 accordingly. As a result, the
 pressure in the loading chamber 12 matches that of the transfer chamber
 16. Every time that the slit valve 22 is open between the chambers, the
 control module 60 calibrates the pressure sensors 62 and 64 relative to
 each other. The same process is performed every time the slit valve 24,
 26, or 28 is open between the transfer chamber 16 and the loading or
 process chamber 14, 18, or 20, respectively. In a configuration of the
 system that utilizes a transfer chamber 16, such as that shown in FIG. 1,
 the pressure sensors for the chambers on either side of the transfer
 chamber are calibrated in reference to the pressure sensor for the
 transfer chamber so that it will be accurate regardless of which chamber
 connecting into it is opened at any one time.
 Referring now to FIG. 2, the reference numeral 80 designates a portion of
 an integrated circuit processing facility with a direct chamber-to-chamber
 connection, including a loading chamber 12, and a process chamber 18. In
 this embodiment, the loading chamber 12 and the process chamber 18 are
 similar to those described in FIG. 1, except that no intermediate transfer
 chamber exists there between. The loading chamber 12 is connected to the
 process chamber 18 through the valve 22. Connected to the loading chamber
 12 is an exhaust line 44 with a variable restriction control valve 52,
 used for controlling the pressure in the chamber. Likewise, connected to
 the process chamber 18 is exhaust line 48 with variable restriction
 control valve 56. Each of the variable restriction control valves 52, 56
 is controlled by a control module 60.
 The control module 60 receives pressure readings from pressure sensors
 (e.g. transducers) 64, 68 connected to the loading chamber 12 and the
 process chamber 18, respectively. Furthermore, the control module 60 is
 informed of the status (e.g. open or closed) of the slit valve 22 through
 the monitor line 72.
 In operation, the control module 60 reads the pressure of the loading
 chamber 12 through the pressure sensor 64 and the process chamber 18
 through the pressure sensor 68. The control module 60 then equalizes the
 pressure in the two chambers 12 and 18 by adjusting the variable
 restriction control valves 52 and 56 accordingly so the pressure in the
 loading chamber 12 matches that of the process chamber 18. Every time that
 the slit valve 22 is open between the chambers, and after the pressure of
 the two chamber has equalized, the control module 60 calibrates the
 pressure sensors 64 and 68 relative to each other.
 Referring to FIG. 3, a calibration method 100 may be performed on a sensor
 inside the system 10 (FIG. 1). For the sake of example, the calibration
 method 100 is used after a wafer 82 is placed in the loading chamber 12
 and is ready to be transferred to the transfer chamber 16. FIG. 4 shows a
 magnified portion of the system 10 for discussion with the present
 example.
 Referring also to FIG. 4, a wafer 82 is in the loading chamber 12, ready
 for transfer to the next chamber 16 through the slit valve 22. At step
 102, it is determined if the wafer 82 is ready for transfer to the next
 chamber. Since in the present example, the wafer is in the loading chamber
 12, step 102 determines that the chamber has been properly closed and
 sealed. At step 104, the control module 60 equalizes the pressure between
 the loading chamber 12 and the next chamber (the transfer chamber, in the
 present example) 16. The control module 60 will not open the slit valve 22
 until the pressure between loading chamber 12 and transfer chamber 16 is
 equalized. Specifically, the control module 60, using readings obtained
 from pressure sensors 62 and 64, will equalize the pressure in the
 transfer chamber 16 and loading chamber 12 by adjusting the variable
 restriction control valves 42 and 52, connected to the exhaust lines 40
 and 44 respectively.
 Referring now to FIGS. 3 and 5, at step 106, when the control module 60
 receives readings from the pressure sensors 62 and 64 that indicate the
 pressures in the loading chamber 12 and transfer chamber 16 are equal,
 execution proceeds to step 108. The control module 60 sends a signal to
 the slit valve 22 to open, as shown in FIG. 5. The wafer 82 will then be
 shifted to the transfer chamber 16 through the slit valve 22.
 While the slit valve 22 is open for the transfer of material, if there is
 any difference in the actual pressure between the loading chamber 12 and
 transfer chamber 16, it will be equalized automatically. At step 110, the
 control module 60 will then check the readings on the pressure sensors 62
 and 64. At step 112, a determination is made as to whether the pressure
 sensors 62, 64 are calibrated relative to each other. If so, then
 execution proceeds to step 114 where the slit valve 22 is closed and
 processing continues on in a normal progression.
 If at step 112 the pressure sensors 62, 64 read differently, execution
 proceeds to step 116 where the sensors are calibrated relative to each
 other. In one embodiment, the pressure sensor 64 is calibrated to the
 pressure sensor 62. By so doing, all of the pressure sensors 64, 66, 68,
 70 (FIG. 1) will eventually be calibrated relative to pressure sensor 62
 and thus to each other. It may be desirable, on a periodic basis, to
 calibrate pressure sensor 62 to a reference measurement. However, even
 this calibration has been simplified because only one sensor needs to be
 calibrated. Execution then returns to step 110. It is understood, however,
 that the control module 60 may have certain error modules so that the
 method will not continually loop to step 110 if the pressure sensors
 cannot be properly calibrated.
 Because the pressure in the two chambers is known to be equal while the
 slit valve 22 is open, calibrating the sensors 62 and 64 relative to each
 other eliminates the potential for a rush of processing fluid and
 constituents between the chambers the next time the valve is open. Also,
 because the process of relative calibration is performed every time the
 slit valve 22 is opened, a variation of no more than 0.01-0.02 Torr should
 occur between any two calibrations. This is an improvement over the prior
 art, where variations of 5 Torr or greater between chambers would be
 common because of the infrequency of calibration.
 Furthermore, because the relative calibration is done during actual use,
 the system does not have to be shut down to perform calibration routines
 quite as often as conventional systems. This, of course, results in
 increased productivity and cycle time.
 Further still, the control module 60 can maintain a history of calibrations
 to determine if any of the pressure sensors must be repeatedly calibrated.
 This may indicate that the particular pressure sensor or corresponding
 chamber is faulty. Likewise, if a majority of the pressure sensors must be
 repeatedly calibrated, this may indicate that the pressure sensor for the
 transfer chamber 16, or corresponding chamber, is faulty.
 Although the invention has been described with reference to specific
 embodiments, such as the manufacture of integrated circuit semiconductors,
 this description is not meant to be construed in a limited sense. The
 invention can be used in a variety of processes. Also, different types of
 valves and chamber may equally benefit from the present invention. Various
 modifications of the disclosed embodiments, as well as alternative
 embodiments of the inventions will become apparent to persons skilled in
 the art upon reference to the description of the invention. It is,
 therefore, contemplated that the appended claims will cover such
 modifications that fall within the scope of the invention.