Abstract:
A mass flow controller has a sensor section that generates an electrical signal, dependent on the measured flow rate. The controller sends a control signal to a magnetic field generating unit, dependent upon the actual flow rate and the desired flow rate, which in response, generates a magnetic flux in the direction of the fluid input to the fluid output through the body of the controller. This means that the magnetic flux is concurrent with the fluid flow within the mass flow controller body. The magnetic flux alters the position of a plunger button assembly, located between the bypass chamber and the fluid output, relative to an orifice plate to control the flow rate to obtain the desired output flow. By incorporating the proportional control valve within the mass flow controller body, the need for a separate and large valve section is eliminated, reducing the size and cost of the controller.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 09/741,552, filed Dec. 19, 2000, now U.S. Pat. No. 6,543,466, which is a continuation-in-part of U.S. patent application Ser. No. 09/517,391, filed Mar. 2,2000, now U.S. Pat. No. 6,314,991. 
    
    
     FIELD OF THE INVENTION 
     The present,invention relates to mass flow controllers. 
     DESCRIPTION OF RELATED ART 
     Mass flow controllers are known in the art for controlling the specific amount of flow of a fluid, necessary for a particular process, e.g., in semiconductor manufacturing processes, such as chemical vapor deposition or the like. Mass flow controllers are known to be capable of sensing the flow occurring through the controller and modifying or controlling that flow as necessary to achieve the required control of the mass of the fluid delivered to the particular process. 
     Sensing the flow is a function of the type of fluid utilized and the physical effect used to sense the amount of flow. One typical type of physical effect to sense mass flow is to measure the temperature differential between the upstream and downstream heater/sensor coils exposed to the fluid flow. Other systems may use absolute and/or differential pressure changes, light absorption, or the momentum change (e.g., paddle wheel) to measure the flow. 
     Modifying or controlling the flow is typically made in response to the sensed flow as it relates to the desired flow by modifying a cross-sectional opening area available to the fluid for flowing. The smaller the area available for flow, the smaller the mass flow, and vice-versa. In the past, this has been accomplished with a typical plunger/diaphragm/orifice system. An orifice provides the variable cross sectional opening area for flow, where the flow control is dictated by the positioning and motion of a plunger/diaphragm or needle stem in the orifice in response to a flow control signal. The flow control signal is generated in response to the measurement of the flow sensor. 
     A servo control section generates a control signal that drives the positioning of the plunger/diaphragm or needle stem, typically through the use of a solenoid type of driver. The solenoid driver has a ferromagnetic core surrounded by a coil. The plunger/diaphragm, typically made of ferromagnetic material, is held close to the orifice by a spring. The energizing of the coil generates a magnetic field that pulls the plunger/diaphragm away from the orifice while the spring pulls it toward the orifice. The distance between the orifice and the plunger/diaphragm is dependent upon the relative strengths of the magnetic field and the spring. The proportional control valve by its nature is not an open and shut valve. The closer the needle stem or plunger/diaphragm is to the orifice, the more restricted the flow becomes, until the flow is shut off, and the more it is withdrawn the more the flow increases, until it no longer affects the amount of flow. 
     For precision control, complex and expensive controller circuitry is needed to control the positioning and movement of the needle stem or plunger/diaphragm as the flow is regulated. The valve parts themselves must be manufactured with high precision, and are therefore expensive. In addition, prior art proportional controlled solenoid valve mass flow controllers require the needle stem or plunger/diaphragm to be mounted at right angles to the fluid flow direction. Consequently, the orifice is also mounted at right angles to the fluid flow path, and the fluid has to change direction to go through the orifice, which generates turbulence in the fluid. 
     Often the mass flow controller, particularly when used in high precision semiconductor manufacturing processes and the like, is part of a tool that has limited space available for the flow controllers, particularly if there are multiple mass flow controllers that are positioned in the immediate area of the actual discharge of the fluid into the tool&#39;s process chamber. 
     There is a need in the art, therefore, for a mass flow controller that is simpler, less expensive, smaller, and easier to manufacture and control. 
     SUMMARY OF THE INVENTION 
     The present invention, according to one embodiment, utilizes a closed loop magnetic flux path passing through the body of the controller in the direction of flow from its input to its output to magnetically operate a flexible plunger button valve assembly that is normally spring biased into the shut position. A current generated from a servo control section of a mass flow controller generates magnetic flux to pull the plunger valve assembly away from an orifice and allow more fluid to flow through. By controlling the amount of flux generated, and thereby the positioning of the button valve assembly relative to the orifice, the flow through the orifice can be controlled. Consequently, a large separate proportional control valve section is no longer necessary, which results in a more compact, less expensive and more reliable mass flow controller that is less costly to manufacture and has fewer components than the conventional mass flow controllers discussed above. 
    
    
     The present invention will be more fully understood upon consideration of the detailed description below, taken together with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a mass flow controller of the prior art; 
     FIG. 1B shows magnetic flux path through a mass flow controller of FIG. 1A; 
     FIG. 2 shows an exploded view of the mass flow controller of FIG. 1A; 
     FIG. 3A shows a mass flow controller according to one embodiment of the present invention; 
     FIGS. 3B-1 and  3 B- 2  show side and front views, respectively, of a bypass assembly of FIG. 3A according to one embodiment; 
     FIGS. 3C-1 and  3 C- 2  show side and front views, respectively, of a second embodiment of a bypass assembly; 
     FIGS. 3D-1 and  3 D- 2  show side and front views, respectively, of a third embodiment of a bypass assembly; 
     FIGS. 3E-1 and  3 E- 2  show side and front views, respectively, of a fourth embodiment of a bypass assembly; 
     FIGS. 3F-1 and  3 F- 2  show side and front views, respectively, of a fifth embodiment of a bypass assembly; 
     FIG. 3G shows a side view of a sixth embodiment of a bypass assembly; 
     FIG. 3H shows magnetic flux path through a mass flow controller of FIG. 3A; 
     FIG. 4 shows an exploded view of the mass flow controller of FIG. 3A; 
     FIG. 5 shows a sectional view of the mass flow controller of FIG. 3A along sectional line A-A′; 
     FIG. 6A shows a side view of the button assembly and orifice plate shown in FIGS. 3A and 4; 
     FIG. 6B shows a side view of the button assembly and orifice plate according to another embodiment; 
     FIGS. 7A and 7B show different configurations of an orifice plate; and 
     FIG. 7C shows a side view of an orifice plate and button plunger assembly; 
     FIG. 8 shows an exploded view of a mass flow controller according to another embodiment of the present invention; 
     FIG. 9 shows magnetic flux path through a mass flow controller according to another embodiment of the present invention; 
     FIG. 10 shows magnetic flux path through a mass flow controller according to yet another embodiment of the present invention; 
     FIG. 11 shows a mass flow controller according to another embodiment of the present invention; 
     FIG. 12 shows the mass flow controller of FIG. 11, rotated 90° about the vertical axis; 
     FIG. 13 shows the mass flow controller of FIG. 11 with securing screws; 
     FIG. 14 shows an exploded view of a portion of the mass flow controller of FIG. 11, rotated 90° about the axis perpendicular to the vertical axis; 
     FIGS. 15 and 16 show a portion of the bypass assembly according to two embodiments of the present invention; 
     FIG. 17 shows a plunger button assembly according to one embodiment; 
     FIG. 18 shows an orifice plate according to one embodiment; and 
     FIG. 19 shows the magnetic flux path through the mass flow controller of FIGS.  11  and  12 . 
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1A and 2 show a conventional mass flow controller  10 . FIG. 1A shows an assembled controller  10 , while FIG. 2 shows an exploded view of parts of controller  10 . Mass flow controller  10  has three main sections: a sensor section  20 , a valve section  30 , and a mass controller block section  40 . A fluid input fitting  11  and a fluid output fitting  12  are sealed to respective input and output ends of block section  40  through metal O-rings  13 . Note that other seals are also suitable, such as knife edge, O-ring, C-ring, and flat gasket, made of materials such as metal, polymer, and elastomer. A cover  14  enclosing sensor section  20  and valve section  30  is secured to input and output fittings  11  and  12  by screws  15 . 
     Gas or fluid enters input fitting  11  through an opening  16  in input fitting  11 . The flow of fluid through mass flow controller  10  is shown in the dark lines in FIG.  1 A. Opening  16  opens into a bypass assembly  17 , which has an input plenum  18  and an output plenum  19 , and which is located within block section  40 . Sensor section  20  is secured to block section  40  via appropriate seals  22 . While a majority of the fluid passes along bypass assembly  17 , a portion of the fluid travels through sensor section  20  along a sensor tube  23 . Bypass assembly  17  restricts the flow of fluid along one of a plurality of channels or grooves formed in the generally cylindrical outer surface of bypass assembly  17  and into output plenum  19 . As is known in the art, this is for the purpose of generating a laminar flow such that a portion of the fluid passing from input plenum  18  into a sensor bypass line  21  and into sensor portion  20  is linearly proportional to the fluid passing from input plenum  18  to output plenum  19  through the plurality of channels or grooves in bypass assembly  17 . 
     Sensor section  20  typically includes multiple coils  24  wrapped around sensor tube  23 . When fluid flows inside sensor tube  23  from a heated upstream coil to a heated downstream coil that are electrically balanced, thermal energy is transferred from the coils to the flowing fluid. The amount of thermal energy transferred from the coils to the fluid is inversely proportional to the fluid temperature. Thermal energy transfer from the upstream coil and the downstream coil to the fluid is disproportionate because the fluid temperature is different at the upstream coil than at the downstream coil. This difference in heat transfer from the upstream coil and the downstream coil results in a temperature differential between the coils which manifests as a change in the relative resistance of the two coils. This change in resistance is directly proportional to the amount of fluid flowing through sensor tube  23 . Typically, a resistor circuit (not shown), which is coupled to the upstream and downstream coils, is configured to form a balanced bridge network when there is no fluid flow. When the fluid flows, the resistance in the coils changes. The bridge network measures the change of the resistance in the coils and generates a signal corresponding to the flow of fluid through sensor tube  23 . 
     Fluid from bypass assembly  17  and sensor tube  23  converge and flow into a fluid flow path  25 . Fluid travels along fluid flow path  25 , through valve section  30 , and out through an opening  26  in output fitting  12 . Valve section  30  includes an upper housing  31  enclosing a wound coil assembly  32  of a solenoid valve, which consists of a pole assembly or plug  33 . Pole assembly  33  has a lower housing  34 , which together with upper housing  31 , are secured to block section  40  and sealed with an O-ring  35  or other appropriate seal. A plunger button assembly  37 , having a flat sealing surface  46 , is held in a cavity in lower housing  34  of pole assembly  33  by a plunger button capture ring  36 . Plunger button capture ring  36 , plunger button assembly  37 , and a plunger button assembly pre-tensioning ring  38  are in abutting relation to an orifice plate  39 , which is sealed to block portion  40  by an O-ring  41  or other appropriate seal. 
     Orifice plate  39  has an opening  42  into which fluid flows from fluid flow path  25 , where the flow of the fluid is controlled by the position of the plunger button assembly  37 , relative to orifice opening  42 . The relative position of plunger button assembly  37  is controlled by magnetic flux generated in core  33  in response to the signal generated from sensor block  20 . Coil  32  is held in place by a top cap  43  and a pole nut  44 . Top cap  43  is sealed with an O-ring  45 . FIG. 1B shows the magnetic flux path of controller  10 . As seen from FIG. 1B, the magnetic flux only travels through valve section  30  to control the position of plunger button assembly  37 , and not through either sensor section  20 , bypass assembly  17 , or block  40 . 
     FIGS. 3A and 4 show a mass flow controller  300  according to one embodiment of the present invention. FIG. 3A shows an assembled controller  300 , while FIG. 4 shows an exploded view of parts of controller  300 . Mass flow controller  300  includes an input fitting  311  attached to an input magnetic flux plate  312 , typically made of ferromagnetic material, where both input fitting  311  and input magnetic flux plate  312  have an opening  313  through which fluid enters and an output fitting  314  attached to an output magnetic flux plate  315 , typically made of ferromagnetic material, where both output fitting  314  and output magnetic flux plate  315  have an opening  316  through which fluid exits. A mass controller block  320 , typically made of non-ferromagnetic material, is sealed between input magnetic flux plate  312  and output magnetic flux plate  315  by O-rings  321  or other appropriate seals, which can be metal, plated metal, polymeric, or elastomeric material. 
     Fluid flows through opening  313  into a bypass assembly  317 , typically formed with a ferromagnetic material, via distribution holes  318 . Bypass assembly  317  can be a single part with longitudinal grooves or channels  350  formed directly thereon, or in other embodiments, bypass assembly can be formed from more than one part, as shown in FIG.  3 B. For example, bypass assembly  317  can be formed from an inner core  355  and an outer sleeve  360  having grooves  350  formed along the outer perimeter. Inner core  355  can be of a ferromagnetic material, while outer sleeve  360  can be of a non-magnetic material. In another embodiment, inner core  355  is made of a non-magnetic material, and outer sleeve  360  is made of a ferromagnetic material. 
     Other embodiments of bypass assembly  317  are shown in FIGS. 3C-1 and  3 C- 2  to  3 F- 1  and  3 F- 2 , and  3 G, where “−1” indicates a side view and “−2” indicates a front view. In each of these embodiments, a bypass assembly  317  includes a ferromagnetic core and pathways along the longitudinal direction of the bypass assembly that allow fluid to flow from one end of the assembly to the other. In FIGS. 3C-1 and  3 C- 2 , ferromagnetic core  355  is surrounded by concentric tubes  361  held in place by ribs  362 . Fluid flows along channels created by concentric tubes  361  and ribs  363 . In FIGS. 3D-1 and  3 D- 2 , ferromagnetic core  355  is surrounded by longitudinal tubes  363  in one or more layers, enclosed by a non-magnetic body  364 . Fluid flows through tubes  363 . In FIGS. 3E-1 and  3 E- 2 , ferromagnetic core  355  is surrounded by one or more laminated sheets  365  having channels  366 , which can be formed by laminating a channeled sheet  367  to a flat sheet  368 . Laminated sheet  365  is then wound around ferromagnetic core  355 . Additional sheets can be wound around an inner sheet to provide multiple channels through which fluid can flow. In FIGS. 3F-1 and  3 F- 2 , ferromagnetic core  355  is surrounded by a porous material  369 , which allows fluid to flow through. In FIG. 3G, core  355  is made of a ferromagnetic porous (sintered) material. Thus, core  355  functions as the path for both the magnetic flux as well as the fluid flow through bypass assembly  317 . 
     Going back to the embodiment of FIGS. 3B-1 and  3 B- 2 , the fluid flows along longitudinal flow groves along the outer circumference of bypass assembly  317 . Fluid also flows through distribution holes  318  to a flow sensor input line  319  formed within block  320 . Input line  319  directs the flow to a sensor unit  322 , which is secured to block  320  by screws  323  and two O-rings  324  or other appropriate seals. One O-ring  324  seals the interface between sensor unit  322  and input line  319  of block  320  and second O-ring  324  seals the interface between sensor unit  322  and an output line  325  formed within block  320 . Fluid from output line  325  and bypass assembly  317  travels through a plunger button assembly capture spacer  326 , typically made of ferromagnetic material, a plunger button assembly  327 , (which includes a plunger made of ferromagnetic material, a spring, and a sealing surface), a plunger button pre-tension spacer  328 , an orifice plate  329  typically made of non-magnetic material, and an orifice metal O-ring  330  or other seal, and out through opening  316  in output fitting  314 . Plunger button assembly  327  and orifice plate  329  are shown in greater detail in FIG.  6 A. Plunger button assembly capture spacer  326  secures plunger button assembly  327 , spacer  328 , orifice plate  329 , and O-ring  330  within a cavity in output magnetic flux plate  315 . 
     In addition, mass flow controller  300  of the present invention includes a magnetic field generating unit  340 . Magnetic field generating unit  340  includes a coil  341  and a core  342  inserted into a cylindrical opening within coil  341 . Core  342  is a cylindrical plug, typically made of a ferromagnetic material, which is inserted into openings in the upper portion of input magnetic flux plate  312  and output magnetic flux plate  315 . Magnetic flux generated by unit  340  is directed down through input magnetic flux plate  312 , to bypass assembly  317 , to plunger button assembly  327 , and back up through output magnetic flux plate  315 . FIG. 3H shows the magnetic flux path of controller  300 . As seen in FIG. 3H, the magnetic flux travels substantially with the fluid flow within the body of controller  300 , i.e., from input magnetic flux plate  312  and through bypass assembly  317  to output magnetic flux plate  315 . This is contrasted with the magnetic flux path of conventional controllers, such as shown in FIG.  1 B. 
     FIG. 5 is a sectional view of mass flow controller  300  along sectional line A-A′ of FIG.  3 A. FIG. 5 shows that sensor unit  322  is rotated approximately 90° from the orientation of conventional mass flow controller  10  shown in FIGS. 1A and 2. In other words, fluid flowing through sensor unit  322  is orthogonal to the flow direction of the fluid through bypass assembly  317  according to the present invention, whereas the flow directions are parallel with the controller shown in FIGS. 1A and 2. Sensor unit  322  is a conventionally known and used thermal mass flow sensor. The majority of the fluid flows through bypass assembly  317  along flow grooves  350  formed longitudinally on the outer surface of bypass assembly  317 . Some of the fluid flows from distribution holes  318  to flow sensor input line  319  and into a flow sensor tube  344 . Sensor tube  344  has wrapped around its outside a first heater/sensor coil  345  and a second heater/sensor coil  346 , which are connected to terminals  347 . 
     Passing current through first coil  345  heats the fluid as it passes through sensor tube  344  in the vicinity of first coil  345 . Current is also passed through second coil  346  wrapped around sensor tube  344  in the downstream flow direction of the fluid, i.e., towards output line  325 . As the fluid passes second coil  346 , it gets hotter. However, the amount of heat transferred from coils  345  and  346  to the fluid is different because the fluid temperature is different at coils  345  and  346 . This in turn changes the relative resistance of coils  345  and  346 , which is measured as a voltage differential in an electrical bridge (i.e., a Wheatstone bridge). This voltage differential corresponds to the mass flow amount of fluid passing through sensor tube  344 , and, proportionately, through bypass assembly  317 . Controller unit  300  includes electronic circuitry, not shown, to calculate the mass flow based upon the sensed change in voltage. A servo control section of controller  300  then generates a current signal for magnetic field generating unit  340 , which in turn generates magnetic flux proportional to the signal to move plunger button assembly  327  to control the flow. The servo control system generates current through the coil to generate sufficient magnetic flux until the error signal is minimized or approximately zero. Such systems are conventional and known to those skilled in the art. 
     FIG. 6A shows, in more detail, plunger button assembly  327  and orifice plate  329  according to one embodiment. Orifice plate  329  is generally flat on both faces, with the face toward button assembly  327  having a frusto-conical portion  600 . Frusto-conical portion  600  has an opening  610  extending through orifice plate  329  such that fluid can flow through orifice plate  329  to opening  316  in output fitting  314 . Plunger button assembly  327  has a smooth flat sealing surface  620  that sits on to frusto-conical portion  600 . Plunger button assembly  327  also has openings  331  located outside sealing surface  620  for fluid to pass through. A spacer  328  (shown in FIG. 4) is positioned between plunger button assembly  327  and orifice plate  329 . Spacer  328  is intended for the purpose of creating an appropriate amount of compression between plunger button assembly  327  and frusto-conical portion  600  by allowing a spring  625  in plunger button assembly  327  to bend to a desired extent by plunger button assembly capture spacer  326 . The thinner the spacer  328 , the greater the bending of spring  625  in plunger button assembly  327 , consequently creating greater compression between plunger button assembly  327  and frusto-conical portion  600 . 
     Fluid flows through openings  331  around the outer edges of surface  620  as well as around the outer edges of plunger button assembly  327  so that fluid can flow from bypass assembly  317  to opening  610  of orifice plate  329 . The amount of fluid flowing into opening  610  depends on the positioning of plunger button assembly  327  in relation to orifice plate  329 . As the attractive force to plunger button assembly  327 , which is created by the magnetic flux, increases, plunger button assembly  327  is moved away from orifice plate  329 , thereby increasing the amount of fluid flowing into opening  610 . However, as the force decreases, the spring pushes button assembly  327  towards orifice plate  329 , thereby decreasing the fluid flow into opening  610 . The spring force of the spring should be as small as possible, yet sufficient to seal opening  610  to give a zero flow through opening  610 . Zero flow means less than 0.5% of the mass flow controller range. 
     FIG. 6B shows another embodiment of plunger button assembly  327  in which a magnet  626  is attached to the side of plunger button assembly opposite sealing surface  620 . By changing the flux direction and magnitude through bypass assembly  317 , plunger button assembly  327  can be moved either away from or towards orifice plate  329 , thereby controlling the flow of fluid through orifice plate  329 . For example, if the magnetic flux creates a pole on the end of bypass assembly  317  that is opposite in polarity to magnet  626 , the attractive force between bypass assembly  317  and plunger button assembly  327  (via magnet  626 ) will pull plunger button assembly  327  away from orifice plate  329 , which allows fluid to flow. If the magnetic flux creates a pole that is the same in polarity as magnet  626 , bypass assembly  317  will force plunger button assembly  327  into orifice plate  329 , which will shut off the fluid flow. Thus, depending on the magnitude and direction of the flux and the strength of magnet  626 , a desired fluid flow can be obtained. 
     In the above described embodiments, opening  610  in orifice plate  329  is a central through hole. However, in other embodiments, opening  610  can be an annular ring of slots  700  (shown in FIG. 7A) or holes  710  (shown in FIG.  7 B), or a combination of both. In these embodiments, the annular ring of holes or slots extend through protruded portions  720  of orifice plate  329 , shown in FIG.  7 C. Plunger button assembly  327  has a central hole  730  or slots (not shown) and sealing surface  740 , which abuts against protruded portions  720  of orifice plate  329 . Without any magnetic flux, protruded portions  720  are sealed against sealing surface  740 , thereby preventing fluid from flowing through the holes or slots in orifice plate  329 . When magnetic flux is generated, plunger button assembly  327  is pulled away from orifice plate  329  to allow fluid flow through orifice plate  329 . Fluid flows through hole  730  of plunger button assembly  327  and holes or slots  750  on the outer edge of sealing surface  740  as well as from the outer perimeter of plunger button assembly  327  to the openings of orifice plate  329 . 
     The size and number of slots  700  or holes  710  can be chosen to make the mass flow controller for a desired flow rate. For a given flow rate, the area of the slots (FIG. 7A) or holes (FIG. 7B) should be minimized to reduce the back pressure, resulting in less force required (less magnetic flux and therefore less current required) to move plunger button assembly  327 . However, this area must not be minimized to the extent that choking occurs when fluid is attempting to pass through orifice plate  329 . Choking can also occur in the peripheral area of the slots or holes. Therefore, the peripheral area of the slots or holes should be greater than or equal to the cross-sectional area of the slots or holes. Referring to FIGS. 7A-7C, the peripheral area can be defined as the perimeter of the slots or holes times a displacement distance d. Distance d is the maximum distance between plunger button assembly  327  and the end of protruded portions  720  for a given flow rate, as shown in FIG.  7 C. 
     Therefore, for a given flow rate and cross-sectional area of slots  700 , the peripheral area of the slots can be made equal to or greater than the cross-sectional area of the slots by either increasing the perimeter of the slots or increasing the distance d. Increasing distance d requires more magnetic force to achieve the desired flow rate. On the other hand, increasing the perimeter of the slots, which can be done by increasing the length of the slots and decreasing the width of the slots, allows the peripheral area of the slots to be increased without changing the cross-sectional area of the slots. Consequently, the back pressure is not adversely increased or affected. However, the same effect cannot be realized by using holes instead of slots because increasing the perimeter or circumference of the holes also increases the cross-sectional area of the holes. 
     FIG. 8 shows another embodiment of the present invention, in which bypass assembly  317  is made of a magneto-restrictive material, instead of a ferromagnetic material described above. The end of bypass assembly  317  facing output magnetic flux plate  315  is secured to a sealing device  800  having holes  805  for fluid to flow through and a sealing area  810  that abuts orifice plate  329  to prevent fluid from flowing through opening  610  in orifice plate  329 . In the normal biased position, sealing device  800  abuts orifice plate  329  when sufficient magnetic flux is generated to seal opening  610 . Magnetic flux travels from input magnetic flux plate  312  toward output magnetic flux plate  315  through bypass assembly  317  and sealing device  800 . When the magnetic flux is reduced, the magneto-restrictive material constricts, which allows fluid to flow through opening  610  in orifice plate  329 . Then, when the magnetic flux is increased, bypass assembly  317  expands until sealing device  800  seals opening  610 . This allows plunger button assembly  327  and plunger button assembly pre-tension spacer  328  of FIG. 4 to be eliminated. 
     In the above described embodiments, the magnetic flux travels through bypass assembly  317 . In other embodiments, shown in FIGS. 9 and 10, the magnetic flux path travels through the body of the mass flow controller. In FIG. 9, the magnetic flux path (shown as a solid black line) travels through core  342 , along input magnetic flux plate  312 , through mass controller block  320 , which in this embodiment is typically made of a ferromagnetic material, through plunger button assembly  327  and back up through output magnetic flux plate  315 . A magnetic flux separator plate or washer  910 , typically made of a non-magnetic material, is located between mass controller block  320  and output magnetic flux plate  315  so that the magnetic flux travels through plunger button assembly  327  to control the fluid flow through orifice plate  329 . In FIG. 10, coil  341  is wound around mass controller block  320 . Mass controller block  320 , typically made of a ferromagnetic material, encloses bypass assembly  317 . An outer cover  100 , typically made of a ferromagnetic material, encloses coil  341  and block  320 . Similar to FIG. 9, magnetic flux separator plate or washer  910  separates mass controller block  320  from output magnetic flux plate  315 . Accordingly, as shown in FIG. 10, the generated magnetic flux (shown as a solid black line) travels through block  320  to plunger button assembly  327 , up through output magnetic flux plate  315 , along outer cover  100 , and down through input magnetic flux plate  312 . Note that in the embodiments shown in FIGS. 9 and 10, fluid flows through sensor section  20  (FIGS. 1A and 1B) parallel to the flow of fluid through bypass assembly  317 . However, the embodiments shown in FIGS. 9 and 10 are also suitable with sensor unit  322  (FIGS. 3A and 5) that allows fluid to flow perpendicular to the flow of fluid through bypass assembly  317 . 
     FIGS. 11-19 show an assembled mass flow controller  920  according to another embodiment of the present invention, with FIG. 14 showing an exploded view of parts of mass flow controller  920 , rotated 90°, from FIG.  11 . Referring to FIG. 11 and 14, mass flow controller  920  has three main sections: a controller block section  921 , a bypass/valve section  922 , and a sensor section  923 . Bypass/valve section  922  with a solenoid core  924  and a solenoid coil  925  are contained within block section  921 . A cover  926  encloses an electronic control printed circuit board (PCB)  927  and sensor section  923 . Mass flow controller  920  is attached and sealed to a surface mount block, such as by screws  928  (FIG. 13) and fluid input/output seals  929 . 
     Referring to FIG. 11, fluid enters through an input port  930  and flows through a channel  931  into an input plenum  932  located within block  921 , which is typically made of a non-ferromagnetic material. There, the fluid is split, with a majority of the fluid flowing along longitudinal grooves/channels  933  (FIGS. 14-16) formed in the generally cylindrical outer surface of a bypass/valve body  934 , typically made from a ferromagnetic material. In various embodiments, grooves/channels  933  can be formed directly on bypass/valve body  934  (FIG.  14 ), on a sleeve  935  (FIG.  15 ), within a sleeve when the sleeve is a porous material that acts as grooves/channels  933 , or on the inner surface of block  921  (FIG.  16 ). Bypass/valve assembly  922 , which includes bypass/valve body  934 , is attached to block  921 , such as by screws  936  (FIG. 13) and seals  937  and  967  (FIGS.  11  and  14 ). Thus, in bypass/valve assembly  922  within block  921 , the fluid flows from fluid input port  930  to fluid input plenum  932  to an output plenum  938 . 
     Referring to FIGS. 11-14 and  19 , sensor section  923  is attached to bypass/valve assembly  922 , such as by screws  939  and seals  940 , and can be mounted in any  3600  orientation substantially perpendicular to the flux path, as shown in FIG.  19 . Sensor section  923  includes conventionally known and used thermal mass flow sensors. Referring to FIG. 12, the smaller portion of the split fluid flows through channel  941  located within bypass/valve body  934  into a sensor tube  942  and exits from sensor tube  942  into channel  943 ,located in bypass/valve body  934  and flows through channel  944  located within block  921 , finally meeting the major portion of the split fluid at the output end of the bypass/valve assembly  922  at output plenum  938 . Sensor tube  942  has wrapped around its outside a first heater/sensor coil  945  and connected to terminals  946 . 
     Passing current through first coil  945  heats the fluid as it passes through sensor tube  942  in the vicinity of first coil  945 . Current is also passed through a second coil  947  wrapped around sensor tube  942  in the downstream flow direction of the fluid, i.e., towards channel  943 . As the fluid passes second coil  947 , it gets hotter. However, the amount of heat transferred from coils  945  and  947  to the fluid is different because the fluid temperature is different at coils  945  and  947 . This in turn changes the relative resistance of coils  945  and  947 , which is measured as a voltage differential in an electrical bridge (e.g., a Wheatstone bridge). This voltage differential corresponds to the mass flow amount of fluid passing through sensor tube  942 , and proportionally through bypass/valve assembly  922 . Mass flow controller  920  includes electronic control PCB  927  to calculate the mass flow based upon the sensed change in voltage. 
     Bypass/valve assembly  922  contains core  924 , typically made from a ferromagnetic material, surrounded by solenoid coil  925 . One end of core  924  is in intimate contact with a valve pole  948 , typically made from a ferromagnetic material. The other end of core  924  is in intimate contact with a solenoid cap  949 , typically made from a ferromagnetic material. Cap  949 , in turn, is in intimate contact with bypass/valve body  934 . Valve pole  948  is separated from bypass/valve body  934  by a flux isolation ring  950 , typically made from a non-ferromagnetic material. 
     An electronic servo control section on PCB  927  generates a current signal (depending upon the actual flow and the desired flow) for solenoid coil  925 , which in turn generates magnetic flux proportional to the signal to move a plunger button assembly  951  (shown in greater detail in FIG. 17) to control the flow, as discussed in more detail below. The servo control system generates current through coil  925  to generate sufficient magnetic flux until the error signal(difference between the desired flow and actual flow) is minimized or approximately zero. 
     An orifice plate  952 , as shown in FIG. 18, typically made of non ferromagnetic material, is generally flat on both faces, with the face towards plunger button assembly  951  having a frusto-conical portion  953 . Frusto-conical portion  953  has an opening  954  extending through orifice plate  952 , such that fluid can flow through orifice plate  952  to a fluid output channel  955  into an output port  956 . Plunger button assembly  951 , as shown in FIG. 17, has a smooth flat sealing surface  957  that sits on to frusto-conical portion  953 . A spring pretension spacer  958  is positioned between plunger button assembly  951  and orifice plate  952 , as shown in FIGS. 11 and 14. Spacer  958  is intended for the purpose of creating an appropriate amount of compression between plunger button assembly  951  and frusto-conical portion  953  by allowing a spring  959  in plunger button assembly  951  to bend to a desired extent by a plunger button capture spacer  960 . The thinner the spacer  958 , the greater the bending of spring  959  in plunger button assembly  951 , consequently creating greater compression between plunger button assembly  951  and frusto-conical portion  953 . 
     From output plenum  938 , fluid flows through grooves/channels  961  (FIG. 18) formed into orifice plate  952  and into opening  954 . The amount of fluid flowing into opening  954  depends on the positioning of plunger button assembly  951  in relation to orifice plate  952 . As the attractive force to plunger button assembly  951 , which is created by the magnetic flux, increases, plunger button assembly  951  is moved away from orifice plate  952 , thereby increasing the amount of fluid flowing into opening  954 . However, as the force decreases, spring  959  pushes plunger button assembly  951  towards orifice plate  952 , thereby decreasing the fluid flow into opening  954 . The regulated fluid from opening  954  then flows through a fluid output channel  955  and exits from output port  956 . 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. For example, the above description describes magnetic flux traveling from the input to the output. However, the magnetic flux can also travel from the output to the input along the direction of the bypass assembly for controlling the fluid flow. The concepts described above can then be modified to open or close the path of the fluid in response to the presence of the magnetic flux. Consequently, various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.