Abstract:
A fault-tolerant system and a method for controlling levels of a fluid in a vessel during a fluid operation, which may include draining the fluid from the vessel or filling the fluid into the vessel, are provided. A first signal indicating the first fluid level in the vessel is sensed by a primary sensor set of the system. A second signal indicating the second fluid level in the vessel is sensed by a secondary sensor set of the system. The fluid operation can be controlled using both the first signal from the primary sensor set and the second signal from the secondary sensor. Alternatively, the fluid operation can be controlled using the second signal from the secondary sensor set if the primary sensor set fails.

Description:
FIELD  
       [0001]     The present invention generally relates to semiconductor integrated circuit technology and, more particularly, to a method and apparatus for supplying process solutions.  
       BACKGROUND  
       [0002]     Conventional semiconductor devices generally include a semiconductor substrate, usually a silicon substrate, and a plurality of sequentially formed dielectric layers such as silicon dioxide and conductive paths or interconnects made of conductive materials. Interconnects are usually formed by filling a conductive material in trenches etched into the dielectric layers. In an integrated circuit, multiple levels of interconnect networks laterally extend with respect to the substrate surface. Interconnects formed in different layers can be electrically connected using vias or contacts.  
         [0003]     The filling of a conductive material into features such as vias, trenches, pads or contacts, can be carried out by electrodeposition. In electrodeposition or electroplating method, a conductive material, such as copper is deposited over the substrate surface including into such features. Then, a material removal technique is employed to planarize and remove the excess metal from the top surface, leaving conductors only in the features or cavities. The standard material removal techniques that are commonly used for this purpose is chemical mechanical polishing (CMP), chemical etching and electropolishing, which is also referred to as electroetching or electrochemical etching.  
         [0004]     In semiconductor processing, equipment reliability is of great importance due to the significant impact it has on total fabrication cost. A great deal of effort is routinely placed on increasing the reliability of the tools employed in semiconductor fabrication. Some steps in semiconductor fabrication require handling of processing or cleaning fluids.  
         [0005]     In wet processes, the electrolytes, etching solutions and various other fluids are used as process fluids. During a process cycle, process fluids are periodically supplied to process modules from fluid tanks. The amount of fluid stored or filled in a fluid tank with a known volume can be determined by sensing the level of the fluid within the tank. Fluid level sensors can be employed to fill a fluid tank up to a predetermined level and to activate a pump to drain the tank once the fluid is reached at the predetermined level.  
         [0006]     The fluid level sensors can be optical, capacitive, conductive, mechanical (floating) or ultrasonic. An exemplary system  10  including a fluid tank  12  with sensors  14  and  15  is illustrated in  FIG. 1 . In system  10 , fluid  16  is supplied from a main tank or a storage tank  18  through a pipe  20 . Pump  22  can be employed to deliver the fluid  16  to the fluid tank  12 . When the fluid  16  reaches the desired level that is detected by the sensor  14 , flow of the fluid  16  into the fluid tank  12  is stopped and the tank can be drained by a drain pump (not shown). The drained solution can be delivered to a process module. As the fluid level in the tank is lowered down to a minimum level detected by sensor  15 , the pump  22  is activated again to fill the tank.  
         [0007]     In operation, conventional fluid level sensors occasionally cause false detection due to a variety of factors. For example, fluid droplets left on or in the vicinity of the sensors cause false readings. Sticking problems in the case of float sensors, or calibration issues of optical and capacitive sensors can also cause false readings with such sensors. Therefore, there is a need for improved reliability of fluid level sensing in such applications. As shown in  FIG. 2A  in enlarged view, in most cases, the sensor signal is generated as soon as the fluid reaches near the surface of the sensor  14  or the surface of the sensor is exposed to the rising fluid. Once the predetermined fill level is detected by the sensor  14 , the fluid flow is stopped and the draining of the tank can be started.  
         [0008]     However, as shown in  FIG. 2B , in such fluid level sensing processes, the fluid droplets  24  or residues can be left on the sensor or in the vicinity of the sensor as the fluid  16  is drained from the tank. These droplets  24  or residues also cause false detection in the subsequent filling of the fluid tank. This failure often brings highly undesirable outcome in applications requiring extreme reliability in fluid level sensing and supplying fluids  
       SUMMARY  
       [0009]     The present invention provides a fault-tolerant system and a method for controlling levels of a fluid in a vessel during a fluid operation such as draining the fluid from the vessel or filling the fluid into the vessel.  
         [0010]     An aspect of the present invention provides a method of controlling levels of a fluid in a vessel during a fluid operation. The fluid operation includes draining the fluid from the vessel or filling the fluid into the vessel. During the process, a first signal indicating at least one first fluid level in the vessel is sensed by a primary sensor set. A second signal indicating at least one second fluid level in the vessel is sensed by a secondary sensor set. The fluid operation is controlled using both the first signal from the primary sensor set and the second signal from the secondary sensor set. Further, the fluid operation is controlled using the second signal from the secondary sensor set if the primary sensor set fails. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic illustration of a prior art fluid level control system;  
         [0012]      FIGS. 2A-2B  are schematic illustrations of prior art sensors during operation;  
         [0013]      FIG. 3  is a schematic illustration of an embodiment of a fluid level sensing system of the present invention;  
         [0014]      FIG. 4  is a schematic view of an embodiment of a control system of the fluid level sensing system of the present invention shown in  FIG. 3 ;  
         [0015]      FIG. 5  is another embodiment of the fluid level sensing system of the present invention;  
         [0016]      FIG. 6  is an algorithm of operation schemes for the embodiment shown in  FIG. 5 ; and  
         [0017]      FIG. 7  is an embodiment of a sensor of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     The present invention provides a fluid sensing method and system to determine fluid levels in fluid vessels such as storage tanks that are used to store process solutions used in the industry. However, although the system and method of the present invention may be used in the field of semiconductor processing, they may be used in any field using or storing solutions. The system of the present invention employs multiple sensors and switches to reliably fill and empty solution vessels. The system of the present invention achieves this by utilizing a series of redundant sensors of the same or different kinds with an appropriate control system.  
         [0019]     In another embodiment of the present invention, the reliability of fluid level sensing process is enhanced in a fluid vessel by protecting the surface of a sensor or surface of such vessel in the immediate vicinity of the sensor from fluid residues by means of a gas pocket contained in a sensor housing. The gas pocket prevents fluid from leaving residues on the sensor so that the sensing action of the sensor is not perturbed by the fluid residues.  
         [0020]      FIG. 3  illustrates an exemplary system  100  including a fluid vessel  102  having sidewalls  103  and a bottom wall  104 . The vessel  102  contains a fluid  105  which may be a process solution such as an electroplating solution or an electropolishing solution, or any other solution. The process solution may be delivered to the vessel  102  using a solution line  107 . A plurality of sensing devices, for example, a first sensor  106 A, a second sensor  106 B, a third sensor  106 C, a fourth sensor  108 A and a fifth sensor  108 B of the present invention are employed to sense various predetermined levels of the solution to activate appropriate fluid operations such as draining or filling processes. The sensors  106 A- 106 C and  108 A- 108 B may preferably be attached to the sidewall  103  of the vessel  102  to sense the predetermined levels of the solution  105  in accordance with the principles of the present invention. The sensors and a drain pump  114  are connected to a control system  118 . The control system  118  activates or deactivates the draining pump  114  based on the input from the sensors.  
         [0021]     In this embodiment, the sensors  106 A- 106 C and  108 A- 108 B are functionally associated with one other in a redundant fashion. For example, a first group of sensors or primary sensors may be configured to include the first sensor  106 A, the second sensor  106 B and the third sensor  106 C. The primary sensors in this embodiment are basically responsible for filling or emptying the solution vessel under normal operation conditions. For example, once the solution is allowed to reach the level of the third sensor  106 C and detected by the third sensor, the draining process is started by the control system. The control system  118  activates the draining pump  114  and drains the solution. The level that is detected by the third sensor  106 C to start draining will be referred to as first predetermined high solution level. During the draining, once the solution reaches the level of second sensor  106 B and this is detected by the second sensor, the draining pump is stopped by the control system  118 . The second sensor  106 B detects a predetermined low solution level in the vessel during a draining. As will be explained below, it is almost a standard procedure to leave some solution in the vessel so as not to dry-run the draining pump, which may cause unwanted effects including but not limited to air bubbles in the solution or damaging the pump or reducing its life. The first sensor  106 A is positioned below the second sensor  106 B and is connected to the second sensor  106 B for redundancy purposes. In this embodiment, if the second sensor  106 B happens to fail, the draining of the solution continues down to another predetermined low solution level which is detected by the first sensor  106 A and the draining is stopped.  
         [0022]     Referring to  FIG. 3 , a second group of sensors or secondary sensors may be configured to include the fourth sensor  108 A and the fifth sensor  108 B. As will be described more fully below, the secondary sensors  108 A,  108 B establish a back-up system for the primary sensors  106 A- 106 C. If the primary sensors fail, the secondary sensors drive the filling and draining process through the control system  118 . The fourth sensor  108 A may detect a third predetermined low solution level when the secondary sensors are needed as backup. The third predetermined low solution level may be equal to or lower than the second predetermined low solution level that is determined by the second sensor  106 B of the primary sensors. The fifth sensor  108 B may detect a second predetermined high solution level to start the draining of the vessel if the primary sensors fail.  
         [0023]     As shown in  FIG. 3 , a signal source  110  is also included in the fluid vessel and located adjacent the bottom wall of the vessel. The signal source  110  emits a signal that is received by the sensors when the fluid level is at the level of a particular sensor, thereby detecting the level of the fluid. In this embodiment, the fluid is an electrically conductive fluid such as an electrolyte or an etching solution. In this respect, sensors of choice are conductive sensors. In one embodiment, the sensor material is preferably titanium-coated platinum. With the conductive sensors, the signal source  110  is a conductive probe that is connected to the electrical ground and exposed to the conductive solution. In this respect, when the solution touches the sensors, a conductive path is established between the sensors and the ground. Conductive path between the ground and a sensor provides a solution level input to the control system. Similarly, termination of this conductive path also show that the solution is no longer at that level.  
         [0024]     An exemplary operation process to fill and drain the fluid vessel  102  with a conductive fluid or process solution using the control system  118  and above described conductive sensor configurations will be described with reference to  FIG. 4 . In  FIG. 4 , each above-mentioned predetermined low or high solution levels and the ground level are shown using dotted lines extending from the corresponding sensors and the signal probe. As exemplified in  FIG. 4 , the control system  118  may comprise a first controller  130 A, a second controller  130 B and a third controller  130 C. In this embodiment, the first and second controller  130 A and  130 B are associated with the primary sensors  106 A- 106 C while the third controller  130 C is associated with the second sensors  108 A and  108 B. Controllers  130 A- 130 C receive input signals from the sensors and the signal source. In this embodiment, the input signals are in the form of electrical currents. As will be described more fully below, upon receiving the input signals, the controllers control a first pump switch  132 A which is a solenoid valve in this embodiment for a pneumatically actuated pump and a second pump switch  132 B or solenoid valve. The first switch  132 A controls the pump, i.e., turns on or off the pump to start or stop draining, through the input from the primary sensors while the second switch controls the pump  114  through the input from the secondary sensors.  
         [0025]     As described above, the sensors are conductive sensors and in this embodiment the electrical source is low voltage AC. The ground probe is connected to each controller  130 A,  130 B and  130 C. As mentioned before there is always some solution left in the vessel under normal conditions to protect the pump  114  and not to let gas bubbles form in the solution  105  after the draining process. This level is determined by one of the low solution levels in which the sensors  106 A,  106 B or  108 A is already shorted with the ground through the solution  105  and provides input for the control system  118 . This level is preferably the second predetermined low level by the second sensor  106 B. However, in order to describe how the control system controllers function, the process will be described as if the vessel is empty at the beginning of the process.  
         [0026]     In  FIG. 4 , the controllers in the control system are presented in a simplified schematic. For this representation each controller has two inputs, which are In 1 , and In 2 , and one output O p . The output O p  of each controller is provided by an output switch or contact switch S- 1 , S- 2  and S- 3  that are closed when both of its inputs In 1 , and In 2  are turned on. However, once closed the output switch S- 1 , S- 2  or S- 3  will open only after both inputs In 1 , and In 2  are turned off again. Furthermore for this representation, each sensor, whether primary or secondary, is on or active when current flows through it. Accordingly, as the solution  105  starts filling the vessel  102  from the bottom wall  104 , the solution  105  first comes into contact with the ground probe  110  and then the solution is connected to the ground of the first, second and third controllers  130 A,  130 B and  130 C. The first sensor  106 A is connected to both inputs In 1 , In 2  of the controller  130 A. As such, when the solution  105  reaches the first predetermined low solution level of the first sensor  106 A, current flows between the first sensor  106 A and the ground through the solution  105  and as a result both inputs In 1 , In 2  of the first controller  130 A are turned on and therefore the output switch S- 1  on the first controller  130 A is closed.  
         [0027]     Closing of the output switch S- 1  of the first controller  130 A connects the second sensor  106 B to the input In 2  of the second controller  130 B. When the solution  105  reaches the second sensor  106 B and provides the second sensor  106 B with a current path to the ground, current flows through the now closed output switch S- 1  and leaves the first controller  130 A as the output O p . The output O p  of the first controller  130 A turns on the input In 2  of the second controller  130 B. At this instant, the output switch S- 2  of the second controller  130 B is still open. At the first predetermined high solution level, electrical connection between the third sensor  106 C and the ground is established, and as a result the input In 1 , of the second controller  130 B is also turned on. Since both inputs In 1 , and In 2  of the controller  130 B are on, the output switch S- 2  of the controller  130 B closes and transmits the output O p  to the first pump switch  132 A. The output O p  of the controller  130 B in this embodiment directly actuates the first pump switch  132 A of the pump  114 . Thereafter, the pump begins draining solution from the vessel  102 .  
         [0028]     The draining process first turns off the In 1  of the second controller  130 B by interrupting the connection between the ground and the third sensor  106 C. Once the level of the solution goes just below the second predetermined low solution level, current flow from the second sensor  106 B to the input In 2  of the second controller  130 B is also interrupted. Since both inputs of the second controller are off, the output switch S- 2  turns off as well, which results in turning off the first pump switch  132 A and the pump  114 .  
         [0029]     One of the fault-tolerant aspects of the present invention may be described with the following example. In the above process, for example, if a malfunction happens and the input In 2  of the second controller  130 B stays on after the fluid level is dropped below the second low solution level  106 B, the pump  114  continues draining because the output switch S- 2  of the second controller  130 B is still on. However, as soon as the solution goes below the first predetermined low solution level of the first sensor  106 A, both inputs In 1 , and In 2  of the first controller  130 A are turned off. This results in opening the switch S- 1  of the first controller  130 A and turning off the input In 2  of the second controller  130 B. Since the inputs In 1 , and In 2  of the second controller  130 B are off, the output switch S- 2  opens and turns off the first pump switch  132 A to stop draining.  
         [0030]     As described above, the secondary sensors ( 108 A and  108 B) provide a back-up system for the primary sensors ( 106 A- 106 C) if the primary sensors fail. Referring to  FIG. 4 , in one embodiment, the secondary sensors function in the following redundant manner. The sensors  108 A and  108 B are activated in the same manner that the primary sensors are activated, i.e., by establishing a current flow between the sensor and the ground through the solution. Beginning of the process, the output switch S- 3  of the third controller  130 C is connected to an output of a power supply and the output switch is in off state. During above-described filling process, the fourth sensor  108 A is also activated at the third predetermined low solution level, along with the first and second  106 A,  106 B of the primary sensors. This turns on the input In 2  of the third controller  130 C. At this point, for example, if the third sensor  106 C of the primary sensors fail and the draining is not activated, solution keeps rising towards the second predetermined high solution level of the fifth sensor  108 B. Contact between the solution  105  and the fifth sensor  108 B turns on the input In 1 , of the third controller  130 C. This closes the output switch S- 3  and the output O p  of the switch S- 3  turns on the second pump switch  132 B to start draining. Similar to the embodiment performed with the primary sensors, draining first turns off the input In 1 , and then just below the third predetermined solution level the input In 2  is turned off. This opens the output switch S- 3  and stop draining at this level. Further, in another aspect of the present invention, if the failure of the primary sensors is remedied, the control system automatically switches back to operate between levels associated with sensors  106 B and  106 C as it is appreciated from the previous description.  
         [0031]     In another embodiment of the present invention, a control system using at least one sensor to detect low solution level and at least one sensor to detect high solution level will be described with help of a control logic of the control system. As shown in  FIG. 5 , the system  200  includes a vessel  202 , a pump  204  to drain a solution from the vessel and a control system  206 . The control system may be a PLC or a microcontroller. In this embodiment, the control system  206  includes four sensors a first low solution level sensor L 1 , a second low solution level sensor L 2 , a first high solution level sensor H 1  and a second high solution level sensor H 2 . The sensors L 1 , L 2 , H 1  and H 2  provide signals to the controller and the controller turns on and of the pump  204  via switch  208 .  
         [0032]      FIG. 6  illustrates an algorithm of operation schemes for the vessel  202  by using the sensors L 1 , L 2 , H 1  and H 2 . For example, as the vessel is filled with a solution, if the sensors L 1  or L 2  and one of H 1  or H 2  is activated, i.e. generate signals for control system. The control system  206  generates an output signal and turns the switch  208  on, which turns on the drain pump and drains the solution from the vessel. Further, if the output from the control system is on and signals from the sensors L 1  and L 2  are still on, the output signal remains on, which keeps the pump running. However, if the output from the control system is off but the signals from the sensors L 1  and L 2  are on, the switch  206  remains off, thus turning off the pump. Further, if the signals from the sensors H 1  and H 2  are both on, even though the signals from the sensor L 1  or/and L 2  are off, the switch still becomes on because of a failure in sensors L 1  and L 2 .  
         [0033]      FIG. 7  illustrates an exemplary sensing device  300  of the present invention which is attached to outer surface of the side wall  302  of a vessel (not shown). The sensing device  300  includes a housing  304  having an inner cavity  306  which is connected to an opening  308  in the side wall  302  through a channel region  310  of the inner cavity. The opening  308  connects the inner cavity  306  of the sensing device  300  to the vessel, thus the fluid flows into the inner cavity through the opening  308 . In this embodiment, a sensor  312  inserted into the inner cavity. The sensor  312  may be a capacitive sensor or optical sensor or other. Surface  314  of the sensor  312  is exposed in the inner cavity  306 . As the fluid level is increased, an air pocket  316  forms below the sensor  312  in the inner cavity and does not allow fluid to wet the surface  314  of the sensor  312 .  
         [0034]     Although various preferred embodiments and the best mode have been described in detail above, those skilled in the art will readily appreciate that many modifications of the exemplary embodiment are possible without materially departing from the novel teachings and advantages of this invention.