Gas gauge proximity sensor with internal gas flow control

Abstract

A choked-flow orifice 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 gas gauge proximity sensor of the present invention replaces the use of a mass flow controller with a choked flow orifice. The use of a choked flow orifice provides for reduced equipment cost and improved system reliability. A gas supply forces gas into the proximity sensor. The gas is forced through the choked flow orifice to achieve sonic conditions at which time the mass flow rate becomes largely independent of pressure variations. The flow of gas proceeds from the choked flow orifice into a sensor channel system. A mass flow sensor within the sensor channel system monitors flow rates to detect measurement standoffs that can be used to initiate a control action.

1 . A gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff, comprising: 
 a gas supply that injects gas into the gas gauge proximity sensor;    a choked flow orifice coupled to the gas supply that chokes the flow of gas; and    a sensor channel system coupled to the choked flow orifice that detects the difference in standoffs between the reference and measurement surfaces at a high sensitivity.    
   
   
       2 . The gas gauge proximity sensor of  claim 1 , wherein the sensor channel system includes: 
 a junction that combines gas flow into a channel coupled to the choked flow orifice of the 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 evenly restricts gas flow through the reference channel;    a second restrictive element located in the measurement channel, wherein said second restrictive element evenly 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.    
   
   
       3 . The gas gauge proximity sensor of  claim 2 , wherein said first and second restrictive elements comprise a porous restrictor or orifice.  
   
   
       4 . The gas gauge proximity sensor of  claim 1 , wherein the gas comprises air.  
   
   
       5 . A method for sensing a difference in a reference standoff and a measurement standoff, comprising the steps of: 
 (a) injecting a flow of gas into a proximity sensor having a measurement channel and reference channel;    (b) forcing the flow of gas through a choked flow orifice to achieve sonic conditions within the flow of the gas;    (c) distributing a flow of gas between a measurement channel and a reference channel;    (d) outputting gas from the reference and measurement channels through nozzles to impinge upon a reference surface and a measurement surface, respectively; and    (e) sensing a mass flow rate across a bridge channel that connects the reference and measurement channels, the mass flow rate being representative of the magnitude of a difference between a measurement standoff and a reference standoff.    
   
   
       6 . The method of  claim 5 , wherein step (c) comprises restricting the flow of gas substantially evenly across cross-sectional areas of both the measurement and reference channels.  
   
   
       7 . The method of  claim 5 , wherein step (e) 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 the magnitude of the difference between the measurement standoff and the reference standoff.  
   
   
       8 . The method as in  claim 7 , further comprising performing a control action in response to said sensing step.  
   
   
       9 . The method of  claim 5 , wherein step (e) comprises the step of monitoring gas pressure differences in the reference and measurement channels, the gas pressure differences being representative of the magnitude of the difference between the measurement standoff and the reference standoff.  
   
   
       10 . The method as in  claim 9 , further comprising performing a control action in response to said sensing step.  
   
   
       11 . The method as in  claim 5 , further comprising performing a control action in response to said sensing step.  
   
   
       12 . A gas gauge proximity sensor for sensing a difference between a reference surface standoff and a measurement surface standoff within a lithography apparatus, comprising: 
 a gas supply that injects gas into the gas gauge proximity sensor;    a choked flow orifice coupled to the gas supply that chokes the flow of gas; and    a sensor channel system coupled to the choked flow orifice that detects the difference in standoffs between the reference and measurement surfaces at a high sensitivity.    
   
   
       13 . The gas gauge proximity sensor of  claim 12 , wherein the sensor channel system includes: 
 a junction that combines gas flow into a channel coupled to the choked flow orifice of the 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 evenly restricts gas flow through the reference channel;    a second restrictive element located in the measurement channel, wherein said second restrictive element evenly 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.    
   
   
       14 . The gas gauge proximity sensor of  claim 13 , wherein said first and second restrictive elements comprise a porous restrictor or orifice.  
   
   
       15 . The gas gauge proximity sensor of  claim 12 , wherein the gas comprises air.