Vacuum driven proximity sensor

Abstract

A vacuum-driven gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff is disclosed. Unlike existing proximity sensors, the vacuum-driven gas gauge proximity sensor uses a vacuum to reverse the traditional flow of gas through a proximity sensor, such that gas flows inward across measurement and reference standoffs through measurement and reference nozzles. The conditioned ambient gas that is vacuumed into the reference and measurement nozzles flows through reference and measurement channels that are coupled at a junction into a single channel. The single channel is coupled to the vacuum that is used to evacuate the conditioned ambient gas through the proximity sensor. A bridge channel couples the reference and measurement channels. A mass flow sensor along the bridge channel monitors flow rates to detect measurement standoffs that can be used to initiate a control action. A pump-driven liquid flow proximity sensor is also disclosed.

1 . A vacuum-driven gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff surrounded by conditioned ambient gas, comprising: 
 a vacuum that evacuates the conditioned ambient gas;    a junction that combines gas flow into a channel coupled to the vacuum of the vacuum-driven gas gauge proximity sensor, wherein the junction combines a reference channel and a measurement channel;    a first resistive element located in the reference channel, wherein said first resistive element restricts gas flow through the reference channel;    a second restrictive element located in the measurement channel, wherein said second restrictive element restricts gas flow through the measurement channel;    a reference probe at an end of the reference channel, whereby gas enters the reference channel through the reference probe having traveled across the reference surface standoff;    a measurement probe at an end of the measurement channel, whereby gas enters the measurement channel through the measurement probe having traveled across a measurement surface standoff; and    a mass flow sensor coupled between the reference and measurement channels that senses the mass of gas flow therebetween, whereby, the difference in standoffs between the reference and measurement surfaces can be sensed at a high sensitivity.    
   
   
       2 . The vacuum-driven gas proximity sensor of  claim 1 , wherein said first and second restrictive elements comprise a porous restrictor or orifice.  
   
   
       3 . The vacuum-driven gas proximity sensor of  claim 1 , further comprising a flow control device located between the vacuum and the junction, wherein said flow control device provides a laminar flow of gas throughout the vacuum-driven proximity sensor.  
   
   
       4 . The vacuum-driven gas proximity sensor of  claim 1 , wherein the vacuum maintains a pressure of about ten pounds per square inch.  
   
   
       5 . A pump-driven liquid flow proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff surround by conditioned ambient liquid, comprising: 
 a pump that evacuates the conditioned ambient liquid;    a junction that combines liquid flow into a channel coupled to the pump, wherein the junction combines a reference channel and a measurement channel;    a first resistive element placed along the reference channel, wherein said first resistive element restricts liquid flow through the reference channel;    a second restrictive element placed along the measurement channel, wherein said second restrictive element restricts liquid flow through the measurement channel;    a reference probe at an end of the reference channel, whereby liquid enters the reference channel through the reference probe having traveled across the reference surface standoff;    a measurement probe at an end of the measurement channel, whereby liquid enters the measurement channel through the measurement probe having traveled across a measurement surface standoff; and    a flow sensor coupled between the reference and measurement channels that senses the flow of liquid therebetween, whereby the difference in standoffs between the reference and measurement surfaces can be sensed at a high sensitivity.    
   
   
       6 . The pump-driven liquid flow proximity sensor of  claim 5 , wherein said first and second restrictive elements comprise a porous restrictor or orifice.  
   
   
       7 . The pump-driven liquid flow proximity sensor of  claim 5 , further comprising a flow control device located between the vacuum and the junction, wherein said flow control device provides a controlled flow of liquid throughout the pump-driven liquid flow proximity sensor.  
   
   
       8 . A method for sensing a difference in a reference standoff and a measurement standoff, wherein the reference and measurement standoffs exist within an environment having conditioned ambient gas, comprising the steps of: 
 (a) evacuating the conditioned ambient gas across reference and measurement standoffs through nozzles coupled to a reference and a measurement channel, respectfully, wherein the measurement standoff is the distance between a measurement nozzle and a measure surface and the reference standoff is the distances between the reference nozzle and reference surface;    (b) restricting the flow of gas substantially evenly across cross-sectional areas of both the measurement and reference channels; and    (c) sensing a flow rate across a bridge channel that connects the reference and measurement channels, the flow rate being representative of the magnitude of a difference between the measurement standoff and the reference standoff.    
   
   
       9 . The method of  claim 8 , wherein step (c) comprises the step of monitoring the mass flow rate across a bridge channel that connects the reference and measurement channels, the mass flow rate being representative of a magnitude of a difference between the measurement standoff and the reference standoff.  
   
   
       10 . The method of  claim 8 , wherein step (c) comprises the step of monitoring gas pressure differences in the reference and measurement channels, the gas pressure differences being representative of a magnitude of a difference between the measurement standoff and the reference standoff.  
   
   
       11 . The method as in  claim 8 , further comprising performing a control action in response to said sensing step.  
   
   
       12 . A method for sensing a difference in a reference standoff and a measurement standoff, wherein the reference and measurement standoffs exist within an environment having conditioned ambient liquid, comprising the steps of: 
 (a) evacuating the conditioned ambient liquid across reference and measurement standoffs through nozzles coupled to a reference and a measurement channel, wherein the measurement standoff is the distance between a measurement nozzle and a measure surface and the reference standoff is the distances between the reference nozzle and reference surface;    (b) restricting the flow of liquid substantially evenly across cross-sectional areas of both the measurement and reference channels; and    (c) sensing a mass flow rate across a bridge channel that connects the reference and measurement channels, the mass flow rate being representative of a magnitude of a difference between the measurement standoff and the reference standoff.    
   
   
       13 . The method of  claim 12 , wherein step (c) comprises the step of monitoring the flow rate across a bridge channel that connects the reference and measurement channels, the flow rate being representative of a magnitude of a difference between the measurement standoff and the reference standoff.  
   
   
       14 . The method as in  claim 13 , further comprising performing a control action in response to said sensing step.  
   
   
       15 . A vacuum-driven gas gauge proximity sensor having a reference and measurement channel for sensing a difference between a reference surface standoff and a measurement surface standoff surrounded by conditioned ambient gas comprising a vacuum that evacuates the conditioned ambient gas across the reference surface and measurement surface standoffs through the reference and measurement channels.  
   
   
       16 . The vacuum-driven gas gauge proximity sensor of  claim 15 , further comprising: 
 a junction that combines gas flow into a channel coupled to the vacuum of the vacuum-driven gas gauge proximity sensor, wherein the junction combines a reference channel and a measurement channel;    a first resistive element located in the reference channel, wherein said first resistive element restricts gas flow through the reference channel; and    a second restrictive element located in the measurement channel, wherein said second restrictive element restricts gas flow through the measurement channel.    
   
   
       17 . The vacuum-driven gas proximity sensor of  claim 16 , wherein said first and second restrictive elements comprise a porous restrictor or orifice.  
   
   
       18 . The vacuum-driven gas proximity sensor of  claim 16 , further comprising a flow control device located between the vacuum and the junction, wherein said flow control device provides a laminar flow of gas throughout the vacuum-driven proximity sensor.  
   
   
       19 . A lithographic leveling system, comprising proximity sensor of  claim 1 ,  5 , or  15 .  
   
   
       20 . A lithographic apparatus, comprising a proximity sensor of claims  1 ,  5  or  15 .