Abstract: There is provided an apparatus for polishing a substrate, including (a) a polishing pad formed with a plurality of through-holes through which polishing material is supplied to a surface of the polishing pad, (b) a level block on which the polishing pad is mounted, and (c) a rotatable carrier for supporting a substrate thereon, the carrier being positioned in facing relation with the level block, the level block being rotatable around a rotation axis thereof with the rotation axis being moved along an arcuate path, and causing the polishing pad to make contact with the substrate for polishing the substrate, the polishing pad having a first ring-shaped region concentric thereto where no through-holes are formed. For instance, the first ring-shaped region has a width greater than 10%, but smaller than 95% of a radius of the polishing pad. The apparatus enhances uniformity in polishing a substrate.

Abstract: Polishing pads with a window, systems containing such polishing pads, and processes that use such polishing pads are disclosed. In embodiments, a polishing pad includes a backing layer having an opening, a polishing layer having an opening aligned with the opening in the backing layer, a solid window of a first material in the opening of the polishing layer, a layer of a first adhesive material between the backing layer and the solid window, and a layer of a second adhesive material between the layer of the first adhesive material and the window.

Abstract: The invention is directed to chemical-mechanical polishing pads comprising a transparent window comprising a polymer resin having a first index of refraction and an inorganic material having a second index of refraction. The transparent window has a light transmittance of about 10% or more at a wavelength of about 200 nm to about 10,000 nm. The difference between the first index of refraction and the second index of refraction is about 0.3 or less at the wavelength.

Abstract: A wafer is pressed against a fixed abrasive and brought into sliding contact with the fixed abrasive. Thus, the wafer is polished. A surface of the fixed abrasive is dressed so as to generate free abrasive particles thereon. A liquid or a gas, composed of a mixture of liquid or inert gas and pure water or chemical liquid, is ejected onto the surface of the fixed abrasive during or after the dressing process.

Abstract: There are provided a polishing apparatus and a polishing method capable of performing polishing a work (such as a wafer) with high efficiency and high precision, a novel work holding plate effectively holding a work and an adhering method for a work capable of adhering the work on the work holding plate with high precision. The polishing apparatus comprises: a polishing table(29); and a work holding plate(38), wherein a work held on the work holding plate(38) is polished supplying a polishing agent solution(41) in the apparatus, and in polishing action, an amount of deformation of the polishing table(29) in a direction normal to an upper surface thereof with respect to the upper surface thereof and/or an amount of deformation of the work holding plate(38) in a direction normal to a work holding surface thereof is restricted to 100 &mgr;m or less by forming the polishing table(29) in one-piece, contriving flow paths of cooling water and others.

Abstract: In polishing particles each having a core-shell structure, a polishing rate can be controlled by adjusting a thickness and/or density of a shell portion. The polishing particles have the core-shell structure with an average diameter in the range from 5 to 300 nm, and the shell portion of the polishing particles includes silica with a thickness in the range from 1 to 50 nm. A density of the shell portion is in the from 1.6 to 2.2 g/cc, while the Na content of the shell portion is less than 10 ppm.

Abstract: In a system and a method according to the present invention, a sensor signal, such as a motor current signal, from a drive assembly of a pad conditioning system is used to estimate the status of one or more consumables in a CMP system.

Abstract: A CMP apparatus is provided for polishing wafers that are substrates to be polished by CMP. The CMP apparatus comprises a stage that is structured to be rotatable and holds a wafer to be polished, a polishing head holding section that holds a polishing head equipped with a polishing pad over the stage, a polishing head storage section that stores replacement polishing heads equipped with polishing pads; and a polishing head replacement mechanism that replaces the polishing head held by the polishing head holding section with the replacement polishing heads stored in the polishing head storage section.

Abstract: A slurry feeding apparatus includes closed slurry bottle, piping, wet nitrogen generator, wet nitrogen supply pipe, suction and spray nozzles, temperature regulator, flow rate control valves, slurry delivery pump and controller for controlling the operation and flow rate of the slurry delivery pump. While a wafer is being polished by a CMP polisher, the controller continuously operates the pump. On the other hand, while the polisher is idling, the controller starts and stops the pump intermittently at regular intervals. No stirrer like a propeller is inserted into the slurry bottle, but the slurry is stirred up by spraying the slurry through the spray nozzle.

Abstract: A polishing pad for a chemical mechanical polishing apparatus. The polishing pad includes a plurality of concentric circular grooves. The polishing pad may include multiple regions with grooves of different widths and spacings.

Abstract: A polish head for Chemical Mechanical Polishing includes a backing film of silicone on a rigid support element, preferably, of amorphous ceramic. The silicone backing film is fabricated by molding, thereby enabling an appropriate cross-sectional shape for specific polishing needs. The head provides a uniform polishing of a semiconductor wafer.

Abstract: A method and system for reclaiming virgin test wafers by polishing a very thin layer from the wafer surface, applying a low down force between the wafer and the pad, with a dilute, low basic slurry. By polishing only a few hundred Angstroms of silicon from the wafer surface, a virgin test wafer may be repeatedly reclaimed and reused for periodic defect monitoring.

Abstract: A pad of an embedding tool is received on a faceplate. The pad is supported on a support member. When the embedding tool is urged against the surface of the faceplate, the pad is allowed to change the attitude relative to the support member in response to undulation generated on the surface of the faceplate. The movement of the flat surface follows the undulation of the faceplate. When a relative movement is induced between the embedding tool and the surface of the faceplate, the surface of the faceplate thus reliably receives the uniform or constant contact of the flat surface. When grains are supplied between the flat surface of the pad and the surface of the faceplate, the grains are uniformly pushed into the surface of the faceplate. In this manner, the grains are uniformly and constantly embedded into the surface of the faceplate.

Abstract: A polishing pad (10) has an upper-layer pad (11) having a hole (11a) defined therein, a light-transmittable window (41) disposed in the hole (11a) for allowing light to pass therethrough, and a lower-layer pad (12) disposed below the upper-layer pad (11) and having a light passage hole (12a) defined therein which has substantially the same diameter as the hole (11a) in the upper-layer pad (11). A transparent film (13) with an adhesive agent applied to upper and lower surfaces thereof is interposed between the upper-layer pad (11) and the lower-layer pad (12). The hole (11a) defined in the upper-layer pad (11) and the light passage hole (12a) defined in the lower-layer pad (12) have substantially the same size as each other.

Abstract: A chemical mechanical polishing apparatus includes a polishing pad. A pad conditioner includes a static conditioner head having a surface area configured to contact and condition the pad. The surface area has a first end proximate to an axis of rotation of the pad and a second end remote from the axis of rotation of the pad. The first end defines a first arc length, and the second end defines a second arc length, where the first arc length and the second arc length are substantially identical.

Abstract: A polishing fluid delivery apparatus has been provided that in one embodiment includes a support member, a dispense arm, a polishing fluid delivery tube and a variable restricting device. The dispense arm extends from an upper portion of the support member and has an outlet of the delivery tube coupled thereto. The restricting device interfaces with the delivery tube and is adapted to provide a variable restriction to flow passing through the delivery tube. In another embodiment, the restricting device is a pinch valve and the tube in continuous from the outlet to beyond a portion that interfaces with the pinch valve. In yet another embodiment, the position of the delivery arm is controllable.

Abstract: The invention provides chemical-mechanical polishing systems, and methods of polishing a substrate using the polishing systems, comprising (a) an abrasive, (b) a liquid carrier, and (c) a positively charged polyelectrolyte with a molecular weight of about 15,000 or more, wherein the abrasive comprises particles that are electrostatically associated with the positively charged polyelectrolyte.

Abstract: An apparatus for use in a chemical mechanical planarization system is provided. The apparatus includes a fluid displacing device and a fluid delivery device. The fluid displacing device is capable of being positioned at a proximate location over a polishing pad, the fluid displacing device configured to displace at least part of a first fluid from a region of the polishing pad. The fluid delivery device is capable of replacing the displaced first fluid with a second fluid at the region of the polishing pad, the second fluid being different than the first fluid. A method of controlling properties of a film present over a polishing pad surface is also provided.

Abstract: A polishing apparatus and a polishing method which can reduce the amount of consumption of the polishing agent and scratches caused on the object to be polished due to an increase of the particle size in the polishing agent. The polishing apparatus has a polishing head which is provided with a polishing agent receiving unit, a holding means for holding an object to be polished in a holding recess, a contact portion which is positioned at the periphery of the holding recess, a feed port for feeding the polishing agent from the polishing agent receiving portion into the closed space, a vibration imparting means for imparting vibration to the polishing agent, and a valve means provided with a valve for opening and closing the feed port.

Abstract: A process for machining a wafer-like workpiece between two plates, in which material is abraded from the workpiece under the influence of an auxiliary substance supplied and of a pressure acting on the workpiece. In this process, the pressure on the workpiece is significantly reduced and then increased again at least once during the machining of the workpiece, and the supply of the auxiliary substance is reduced as the pressure is increased.

Abstract: A system includes a measuring station for positioning an eddy current probe proximate to a substrate in a substrate holder. The probe can produce a time-varying magnetic field, in order to induce eddy currents in one or more conductive regions of a substrate either prior to or subsequent to polishing. The eddy current signals are detected, and may be used to update one or more polishing parameters for a chemical mechanical polishing system. The substrate holder may be located in a number places; for example, in a substrate transfer system, a factory interface module, a cleaner, or in a portion of the chemical mechanical polishing system away from the polishing stations. Additional probes may be used.

Abstract: A projected gimbal point drive system is disclosed. The projected gimbal point drive system includes a spindle capable of apply a torque, and having a concave spherical surface formed on its lower portion. Further included is a wafer carrier disposed partially within the lower portion of the spindle. The wafer carrier has a convex spherical surface formed on a surface opposite the concave spherical surface of the spindle. In addition, a drive cup is included that is disposed between the spindle and the wafer carrier. The drive cup has a concave inner surface and a convex outer surface, and allows the wafer carrier to be tilted about a predefined gimbal point. In this manner, torque can be applied without affecting the gimbal action.

Abstract: Vending apparatus for optical disk restoration has a housing for receiving the disk form a customer; a disk inspection device to determine whether the disk is correctly positioned for polishing and providing a signal indicative of disk position, a disk perforation detecting device providing a signal indicative of any perforation in a reflective layer of the disk; and a device for ejecting a perforated disk from the housing. A user interface requests and receives payment from a customer in response to a signal by the detecting device that the reflective layer is imperforate and provides a signal indicating receipt of correct payment. Polishing wheels polish, with slurry, an exposed light receiving surface of a data carrying protective layer of the disk to ameliorate scratches in response to a signal from the user interface of correct payment and a device ejects a polished disk from the housing to a customer.

Abstract: A pad conditioner of wafer planarization equipment includes a linear driving device for moving a disk holder up and down using a magnetic force. The linear driving device has an electromagnet, a permanent magnet, and a controller. The controller controls the power supplied to the electromagnet such that the electromagnet and permanent magnet produce a force of attraction and/or repulsion used to move the disk holder relative to a polishing pad of the wafer planarization equipment.

Abstract: Machines with solution dispensers and methods of using such machines for chemical-mechanical planarization and/or electrochemical-mechanical planarization/deposition of microelectronic workpieces. One embodiment of such a machine includes a table having a support surface, a processing pad on the support surface, and a carrier assembly having a head configured to hold a microelectronic workpiece. The carrier assembly can further include a drive assembly that manipulates the head. The machine can also include a solution dispenser separate from the head. The solution dispenser can include a support extending over the pad and a fluid discharge unit or distributor carried by the support. The fluid discharge unit is configured to discharge a planarizing solution onto a plurality of separate locations across the pad.

Abstract: A carrier head for a chemical mechanical polishing apparatus includes a flexible membrane that applies a controllable load to a substrate in an area with a controllable inner diameter.

Abstract: A method and apparatus for processing a microelectronic substrate. In one embodiment, the method can include planarizing the microelectronic substrate with a planarizing liquid and rinsing the substrate with a rinsing liquid having a pH approximately the same as a pH of the planarizing liquid. The rinsing step can be completed while the substrate remains on a polishing pad of the apparatus, or, alternatively, the substrate can be removed to a rinsing chamber for rinsing. In another embodiment, the method can include conditioning the polishing pad by removing polishing pad material from the polishing pad and then cleaning the microelectronic substrate by engaging the substrate with the same polishing pad and moving at least one of the polishing pad and the substrate relative to the other of the polishing pad and the substrate after conditioning the polishing pad.

Abstract: Apparatuses and methods for planarizing a microelectronic-device substrate assembly on a planarizing pad. In one aspect of the invention, material is removed from the substrate assembly by pressing the substrate assembly against a planarizing surface of a planarizing pad and moving the substrate assembly across the planarizing surface through a planarizing zone. The method also includes replacing at least a portion of a used volume of planarizing solution on the planarizing surface with fresh planarizing solution during the planarization cycle of a single substrate assembly. The used planarizing solution can be replaced with fresh planarizing solution by actively removing the used planarizing solution from the pad with a removing unit and depositing fresh planarizing solution onto the pad in the planarizing zone.

Abstract: In a machine for planarizing wafers, when a spindle carrier descends over the load station, it needs a way of determining whether it should descend to a first position suitable for depositing a wafer onto the load station or whether it should descend to a lower second position suitable for acquiring a wafer that is already present on the load station. The present invention provides a way of making this determination. The load station includes three upwardly-directed nozzles for use in supporting a wafer on three separate spaced cushions of purified water. The nozzles are supplied through branch conduits from a supply main. When no wafer is present, the pressurized water meets with little resistance as it is discharged from the nozzles. Accordingly, the pressure in the branches is relatively low. In contrast, when a wafer is present the wafer partially impedes the discharge of the water from the nozzles, causing the pressure in the branches to be greater than when no wafer is present.

Abstract: The invention provides a chemical-mechanical polishing system comprising an abrasive, a carrier, and either boric acid, or a conjugate base thereof, wherein the boric acid and conjugate base are not present together in the polishing system in a sufficient amount to act as a pH buffer, or a water-soluble boron-containing compound, or salt thereof, that is not boric acid, and a method of polishing a substrate using the chemical-mechanical polishing system.

Abstract: A polishing apparatus comprises a first polishing table having a first polishing surface, a substrate carrier for holding a substrate and positioning the substrate so as to bring a surface of the substrate into contact with the first polishing surface, a pressing mechanism for pressing, against the first polishing surface, the surface of the substrate which has been brought into contact with the first polishing surface by the substrate carrier, a retainer ring mounted on the substrate carrier so as to surround the substrate which has been pressed against the first polishing surface by the pressing mechanism, and a retainer-ring-position-adjustment mechanism for adjustably positioning the retainer ring relative to the substrate, which has been pressed against the first polishing surface, in directions toward and away from the first polishing surface.

Abstract: The invention includes a semiconductive processing method of electrochemical-mechanical removing at least some of a conductive material from over a surface of a semiconductor substrate. A cathode is provided at a first location of the wafer, and an anode is provided at a second location of the wafer. The conductive material is polished with the polishing pad polishing surface. The polishing occurs at a region of the conductive material and not at another region. The region where the polishing occurs is defined as a polishing operation location. The polishing operation location is displaced across the surface of the substrate from said second location of the substrate toward said first location of the substrate. The polishing operation location is not displaced from said first location toward said second location when the polishing operation location is between the first and second locations.

Abstract: A method for preparing a semiconductor die for analysis comprises providing a semiconductor die having a connector on one side and an opposite, backside surface to be analyzed, providing a polishing pad for polishing the backside surface of a semiconductor die, providing a rotatable spindle for securing the polishing pad, and providing a constant force actuator on the spindle, the constant force actuator being adapted to provide constant force between the polishing pad and the backside surface of the die. The method then includes contacting the backside die surface with the polishing pad, rotating the spindle and polishing pad, and polishing the backside surface of the die while maintaining the substantially constant force of the polishing pad on the die backside surface with the constant force actuator.

Abstract: An angular substrate polishing method includes the steps of holding an angular substrate having a surface to be polished within a guide ring of a substrate holding head; pressing the substrate surface to be polished, and also one surface of the guide ring, against a polishing pad; and independently rotating the polishing pad and the substrate-holding head together with the substrate it holds while pressing the polishing pad-contacting surface of the guide ring against the polishing pad, to thereby polish the substrate surface. During the polishing step, a pressing force is applied to the guide ring which is separate from the pressing force applied to the substrate, enhancing the flatness of the polished substrate.

Abstract: A slurry feeding apparatus includes closed slurry bottle, piping, wet nitrogen generator, wet nitrogen supply pipe, suction and spray nozzles, temperature regulator, flow rate control valves, slurry delivery pump and controller for controlling the operation and flow rate of the slurry delivery pump. While a wafer is being polished by a CMP polisher, the controller continuously operates the pump. On the other hand, while the polisher is idling, the controller starts and stops the pump intermittently at regular intervals. No stirrer like a propeller is inserted into the slurry bottle, but the slurry is stirred up by spraying the slurry through the spray nozzle.

Abstract: Conductive elements of a chemical mechanical polishing system may generate undesired eddy currents under the influence of a time-dependent magnetic field used in an eddy current monitoring system. To improve the accuracy of an eddy current monitoring system, elements that may contribute an undesired signal to the sensed eddy current signal may be fabricated from a non-conductive material such as plastic or ceramic. In some implementations, elements may be fabricated from non-magnetic materials.

Abstract: A method and apparatus for planarizing a microelectronic substrate. In one embodiment, the apparatus can include an elongated, non-continuous polishing pad oriented at an angle relative to the horizontal to allow planarizing liquids and materials removed from the microelectronic substrate to flow off the polishing pad under the force of gravity. Two such polishing pads can be positioned opposite each other in a vertical orientation and can share either a common platen or a common substrate carrier. The polishing pads can be pre-attached to both a supply roll and a take-up roll to form a cartridge which can be easily removed from the apparatus and replaced with another cartridge.

Abstract: A polishing apparatus comprises a polishing table having a polishing surface thereon, a top ring for pressing a workpiece to be polished against the polishing surface, and a dresser for dressing the polishing surface on the polishing table. The dresser comprises a dressing element provided on a surface of the dresser for dressing the polishing surface by sliding contact with the polishing surface, and an ejection nozzle provided on the surface of the dresser for ejecting a fluid supplied from a fluid source toward the polishing surface.

Abstract: The present invention discloses a polishing pad that can facilitate process stability, extend length of use, and mitigate process non-uniformity and process induced defects for chemical mechanical planarization processes. The polishing pad of the present invention is a composite of a top pad and a sealed sub-pad. The sealed sub-pad has a sealing mechanism that mitigates liquid penetration into the sub-pad thereby maintaining a substantially uniform compressibility of the sub-pad and the polishing pad and extending a useable life of the polishing pad.

Abstract: An apparatus and method for re-surfacing compact discs. The method is a two-step method in which the disc to be re-surfaced is first placed on a shaft. A resilient pad supports an abrasive medium and is positioned so the abrasive medium overlaps only the protective layer of the CD. The disc and abrasive material are rotated in opposite directions at about the same speed to remove deeper blemishes in a wet operation. Subsequent polishing is accomplished in a similar manner using a polishing compound instead of an abrasive. The re-surfacing and polishing apparatus each have motor driven shafts which are parallel and spaced apart and support the CD and pad in a cabinet. A transparent cover extends across the top of the cabinet.

Abstract: An apparatus for the chemical-mechanical polishing of surfaces of circular flat workpieces, in particular semi-conductor wafers, comprising a loading and unloading station for the workpieces which includes a carrier which is supported for rotation about a vertical axis and is driven by a rotary driving means into a predetermined rotary position, at least two horizontal loading surfaces on the carrier means facing upwardly. With a transfer means the workpieces can be placed on the loading surfaces or removed therefrom.

Abstract: Techniques for polishing a wafer (10) include closed-loop control. The wafer can be held by a carrier head (100) having at least one chamber whose pressure is controlled to apply a downward force on the wafer. Thickness-related measurements of the wafer can be obtained during polishing and a thickness profile for the wafer is calculated based on the thickness-related measurements. The calculated thickness profile is compared to a target thickness profile. The pressure in at least one carrier head chamber is adjusted based on results of the comparison. The carrier head chamber pressures can be adjusted to control the amount of downward force applied to the wafer during polishing and/or to control the size of a loading area on the wafer against which the downward force is applied.

Abstract: A continuous CMP process for polishing multiple conductive and non-conductive layers on a semiconductor substrate. The continuous process comprises the steps of: (a) disposing a substrate on a platen; (b) polishing a first layer using both a mechanical means and a chemical means; and (c) polishing a second layer upon adjusting at least one parameter in either the mechanical means, chemical means, or both.

Abstract: A CMP pad conditioning unit having a new and improved drive transmission for transmitting rotation from a drive motor to a conditioning disk on the pad conditioning unit. The drive transmission includes a rotatable transmission shaft that is drivingly engaged by the drive motor and drivingly engages the conditioning disk. In operation, rotation is transmitted from the drive motor to the conditioning disk through the transmission shaft. The drive transmission is characterized by reliability, longevity and enhanced efficiency in the conditioning of substrate polishing pads.

Abstract: Abrasive sheet materials, abrasive sheet materials with island distributions of abrasive particles, processes for manufacture of abrasive sheet materials with minimized abrasive content (with monolayers and as few as four layers of abrasive particles), processes for using the abrasive sheeting in high speed lapping/abrading processes, and apparatus for using the abrasive sheeting are described. The process for manufacturing the abrasive sheeting provides an economical method for providing improved quality sheeting, while also allowing for greater control over the shape and distribution of abrasive islands on the sheet than is available from present processes of manufacture.

Abstract: An apparatus and method for refurbishing fixed-abrasive polishing pads. In one embodiment, a refurbishing device has an arm positionable over the planarizing surface of the polishing pad, a refurbishing element attached to one end of the arm, and an actuator connected to the other end of the arm. The refurbishing element has a non-abrasive contact medium engageable with the planarizing surface of the polishing pad that does not abrade or otherwise damage raised features on the fixed-abrasive pad under desired conditioning down forces. The actuator moves the arm downwardly and upwardly with respect to the planarizing surface to engage and disengage the non-abrasive contact medium with the planarizing surface of the polishing pad. The refurbishing device may also have a conditioning solution dispenser positionable proximate to the planarizing surface of the polishing pad to dispense a liquid conditioning solution onto the planarizing surface.

Abstract: A holder for semiconductor wafers in an apparatus for chemical-mechanical polishing of semiconductor wafers, having a disk-shaped head, a holding plate and a ring-shaped membrane attached to the carrier section and the holding plate which defines a pressure chamber between these components, the bores in the holding plate being connected with the pressure chamber, a contact membrane of elastomeric gas-impermeable material having a peripheral edge which is fixedly connected to a peripheral portion of the holding plate in a gas-tight manner and engages the lower side of the holding plate.

Abstract: Carrier assemblies, polishing machines with carrier assemblies, and methods for mechanical and/or chemical-mechanical polishing of micro-device workpieces are disclosed herein. In one embodiment, a carrier assembly includes a head having a chamber, a magnetic field source carried by the head, and a magnetic fluid in the chamber. The magnetic field source is configured to generate a magnetic field in the head. The magnetic fluid changes viscosity within the chamber under the influence of the magnetic field to exert a force against at least a portion of the micro-device workpiece. The magnetic fluid can be a magnetorheological fluid. The magnetic field source can include an electrically conductive coil and/or a magnet, such as an electromagnet. The carrier assembly can also include a fluid cell with a cavity to receive the magnetic fluid.

Abstract: A polishing pad for polishing a notch on a circumferential edge portion of a wafer has a disk-shaped center pad of a synthetic resin material and holder plates secured to both surfaces of the center pad so as to expose circumferential portions of the center pad. The circumferential portion of the center pad is sectionally shaped so as to be fittingly insertable into the notch. A foamed resin such as foamed urethane is preferably used as the synthetic resin material. The average diameter of air bubbles in the foamed material is in the range of 10 &mgr;m-500 &mgr;m. The hardness of the foamed material is in the range of 25-95 degrees in the Shore hardness A scale. Such a polishing pad is rotated and the circumferential portion of its center pad is fittingly inserted into and pressed against the notch while slurry is supplied to their contact part through a nozzle to polish the notch.

Abstract: The present invention is directed to an apparatus and method for flow regulation of planarization fluids to a semiconductor wafer planarization machine. In one embodiment, the regulating system includes a fluid storage tank with an acoustic fluid level sensor. The storage tank is connected to a fluid delivery line that delivers planarization fluid to the storage tank through a flow control valve and delivers a regulated flow of planarization fluid to a planarization machine through a flow sensor. A gas supply system is connected to the storage tank to provide system pressurization. Regulation of the fluid flow is achieved by a control system in which the flow sensor and the acoustic fluid level sensor comprise feedback elements in a closed feedback system to independently control the pressure in the storage tank and the fluid admitted by the control valve. In an alternate embodiment, the fluid level sensor is comprised of capacitive proximity sensors located outside the wall of the storage tank.