Abstract:
A miniature oxygen sensor makes use of paramagnetic properties of oxygen gas to provide a fast response time, low power consumption, improved accuracy and sensitivity, and superior durability. The miniature oxygen sensor disclosed maintains a sample of ambient air within a micro-channel formed in a semiconductor substrate. O 2  molecules segregate in response to an applied magnetic field, thereby establishing a measureable Hall voltage. Oxygen present in the sample of ambient air can be deduced from a change in Hall voltage with variation in the applied magnetic field. The magnetic field can be applied either by an external magnet or by a thin film magnet integrated into a gas sensing cavity within the micro-channel. A differential sensor further includes a reference element containing an unmagnetized control sample. The miniature oxygen sensor is suitable for use as a real-time air quality monitor in consumer products such as smart phones.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to the fabrication of microelectronic environmental sensors, such as gas composition sensors. 
         [0003]    2. Description of the Related Art 
         [0004]    Oxygen is one of the most important and abundant chemical species. It constitutes 21% of the earth&#39;s atmosphere, and is needed to sustain plant and animal life. Oxygen is also widely employed for industrial purposes. Conventional oxygen sensors are widely used in laboratory and industrial applications, wherever oxygen is consumed, emitted, or otherwise present. Such oxygen sensors, for example, those shown in  FIGS. 2 and 3  below, generally have large scale form factors, tend to be expensive, and typically require involvement of skilled operators. 
         [0005]    With the advent of micro-mechanical systems, deployment of sensors and feedback control systems in smaller scale applications is made possible. For instance, many electronic devices now contain environmental sensors. In one example, electronic thermostats connected to climate control systems rely on temperature sensors to trigger activation of furnaces and air conditioners. In another example, electronic weather stations rely on internal temperature sensors, barometric pressure sensors, and humidity sensors. Small scale consumer products such as motion activated air fresheners can make use of sensors that detect changes in ambient light, or changes in air flow. 
         [0006]    Miniature sensors that are typically embedded on board mobile computing devices such as smart phones include, for example, magnetic field sensors used to determine orientation of the smart phone relative to the earth&#39;s magnetic field. Providing additional environmental sensors within smart phones, tablet computers, and the like may encourage program developers to create applications that otherwise might not be possible. In particular, if miniaturized gas sensors could be provided in consumer products, they might find wide use for medical applications such as pulmonary monitoring. 
       BRIEF SUMMARY 
       [0007]    A miniature gas sensor leverages paramagnetic properties of oxygen gas to measure a Hall voltage induced by gas molecule segregation in an applied magnetic field. Certain atoms, including oxygen, have a net magnetic dipole moment that tends to align in an external magnetic field. While such paramagnetic effects can be canceled out by collisions on a macroscopic scale, they can be sustained more easily across small distances that exist within microelectronic devices. 
         [0008]    The miniature sensor described can be implemented to detect gases other than oxygen, provided the subject gas also exhibits paramagnetism or another property that can be similarly manipulated. 
         [0009]    The miniature oxygen sensor disclosed maintains a sample of ambient air within a micro-channel formed in a semiconductor substrate. The micro-channel contains a constant volume of O 2  gas. When subjected to a magnetic field, the O 2  molecules align with the magnetic field and segregate, thereby establishing a measureable Hall voltage. A concentration of oxygen present in the sample of ambient air can be deduced from a change in Hall voltage in response to at least a momentary variation in the applied magnetic field. The magnetic field can be applied either by an external magnet or by a thin film magnet that is integrated into the micro-channel. 
         [0010]    A differential sensor further includes a second micro-channel containing a control sample to which a magnetic field is not applied. Comparison of the Hall voltage measurement and a control measurement yields a differential Hall voltage measurement having improved accuracy. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    In the drawings, identical reference numbers identify similar elements. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
           [0012]      FIG. 1  is a screen shot of a smart phone running a weather station application that displays data from an on-board miniature oxygen gas micro-sensor. 
           [0013]      FIG. 2  is a tree diagram showing different types of existing oxygen gas detection devices. 
           [0014]      FIG. 3  is a schematic diagram of a conventional macro-scale mechanical gas detection system. 
           [0015]      FIG. 4A  is a schematic diagram of a prior art electrical circuit for use in implementing a Hall effect sensor. 
           [0016]      FIG. 4B  is a plot of Hall voltage measured in response to an applied magnetic field for the Hall effect sensor shown in  FIG. 4A . 
           [0017]      FIG. 5  is a cross section of a prior art integrated circuit implementation of a Hall effect sensor that may be used in the electrical circuit shown in  FIG. 4A . 
           [0018]      FIGS. 6A-6C  are cross sections along a cut line A-A′ (shown in  FIG. 7 ) of an oxygen gas micro-sensor having an external magnet, as described herein. 
           [0019]      FIG. 7  is a top plan view of a differential oxygen gas micro-sensor (left) and a reference sensor (right) as described herein. 
           [0020]      FIG. 8  is a plot of differential output voltage as a function of magnetic field strength for three different gas concentrations, as measured by a gas micro-sensor that uses the Hall effect. 
           [0021]      FIG. 9A  is a reproduction of the cross section shown in  FIG. 6A , in which an external magnet is used to magnetize a gas sensing cavity. 
           [0022]      FIG. 9B  is a cross-sectional view of a micro-sensor in which an integrated magnetic thin film is used to magnetize a gas sensing cavity. 
           [0023]      FIGS. 10A and 10B  are top plan views of an oxygen gas micro-sensor (left) that uses an external magnet, and a similar micro-sensor having an integrated thin film magnet (right). 
           [0024]      FIGS. 11A-11C  are cross-sectional views of an oxygen gas micro-sensor that includes an integrated magnetic thin film, as described herein. 
           [0025]      FIG. 12  is a flow diagram showing a sequence of processing steps in a method of fabricating a gas micro-sensor as described herein. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    It will be appreciated that, although specific embodiments of the present disclosure are described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the present disclosure. Accordingly, the present disclosure is not limited except as by the appended claims. 
         [0027]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of semiconductor processing comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
         [0028]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
         [0029]    Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification are not necessarily all referring to the same aspect. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more aspects of the present disclosure. 
         [0030]    In the following description, certain specific details are set forth in order to provide a thorough understanding of various aspects of the disclosed subject matter. However, the disclosed subject matter may be practiced without these specific details. In some instances, well-known structures and methods of semiconductor processing comprising embodiments of the subject matter disclosed herein have not been described in detail to avoid obscuring the descriptions of other aspects of the present disclosure. 
         [0031]    Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
         [0032]    Reference throughout the specification to insulating materials or semiconducting materials can include various materials other than those used to illustrate specific embodiments of the transistor devices presented. The term “epitaxial silicon compounds” should not be construed narrowly to limit an epitaxially grown structure to Si or SiGe, for example, but rather, the term “epitaxial silicon compounds” is broadly construed to cover any compounds that can be grown epitaxially from a crystalline silicon surface. 
         [0033]    Reference throughout the specification to conventional thin film deposition techniques for depositing silicon nitride, silicon dioxide, metals, or similar materials include such processes as chemical vapor deposition (CVD), low-pressure chemical vapor deposition (LPCVD), metal organic chemical vapor deposition (MOCVD), plasma-enhanced chemical vapor deposition (PECVD), plasma vapor deposition (PVD), atomic layer deposition (ALD), molecular beam epitaxy (MBE), electroplating, electro-less plating, and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. For example, in some circumstances, a description that references CVD may alternatively be done using PVD, or a description that specifies electroplating may alternatively be accomplished using electro-less plating. Furthermore, reference to conventional techniques of thin film formation may include growing a film in situ. For example, in some embodiments, controlled growth of an oxide to a desired thickness can be achieved by exposing a silicon surface to oxygen gas or to moisture in a heated chamber. 
         [0034]    Reference throughout the specification to conventional photolithography techniques, known in the art of semiconductor fabrication for patterning various thin films, includes a spin-expose-develop process sequence involving a photoresist. Such a photolithography sequence entails spinning on the photoresist, exposing areas of the photoresist to ultraviolet light through a patterned mask, and developing away exposed (or alternatively, unexposed) areas of the photoresist, thereby transferring a positive or negative mask pattern to the photoresist. The photoresist mask can then be used to etch the mask pattern into one or more underlying films. Typically, a photoresist mask is effective if the subsequent etch is relatively shallow, because photoresist is likely to be consumed during the etch process. Otherwise, the photoresist can be used to pattern a hard mask, which in turn, can be used to pattern a thicker underlying film. 
         [0035]    Reference throughout the specification to conventional etching techniques known in the art of semiconductor fabrication for selective removal of polysilicon, silicon nitride, silicon dioxide, metals, photoresist, polyimide, or similar materials includes such processes as wet chemical etching, reactive ion (plasma) etching (RIE), washing, wet cleaning, pre-cleaning, spray cleaning, chemical-mechanical planarization (CMP) and the like. Specific embodiments are described herein with reference to examples of such processes. However, the present disclosure and the reference to certain deposition techniques should not be limited to those described. In some instances, two such techniques may be interchangeable. For example, stripping photoresist may entail immersing a sample in a wet chemical bath or, alternatively, spraying wet chemicals directly onto the sample. 
         [0036]    Specific embodiments are described herein with reference to examples of integrated micro-sensors that have been produced; however, the present disclosure and the reference to certain materials, dimensions, and the details and ordering of processing steps are exemplary and should not be limited to those shown. 
         [0037]    In the figures, identical reference numbers identify similar features or elements. The sizes and relative positions of the features in the figures are not necessarily drawn to scale. 
         [0038]      FIG. 1  shows a smart phone  1  equipped with a miniature magnetic field-based gas sensor configured as an oxygen detector that can be used to monitor ambient air quality in real time. The miniature gas sensor, as described below, is designed to have a fast response time, on the order of milliseconds, about a thousand times faster than conventional gas concentration sensors. Furthermore, because the miniature gas sensor relies on a Hall effect sensing element, it consumes very little electrical power, making it feasible for deployment on a mobile computing device. Because the miniature gas sensor contains no reacting material, it has an essentially indefinite lifetime. 
         [0039]    A shell of the smart phone  1  can be modified so as to allow exposure of the capacitive sensors to ambient air. An exemplary smart phone application (“app”) can, for example, be programmed to display on the smart phone screen  2  weather station icons  3 . The smart phone app can report measurements of temperature, relative humidity, pressure, and gas concentration (e.g., concentration of oxygen molecules O 2 ) via the readouts  4 ,  5 ,  6 , and  7 , respectively. The smart phone app can further provide an assessment of air quality  8  based on a comparison of the measurements to a selected standard. The standard can be pre-programmed or set by a user of the smart phone, for example. 
         [0040]    Examples of different types of conventional oxygen gas detectors  10  are shown in  FIG. 2 . These include near infrared (NIR) photodetectors  12 , solid electrolyte detectors  14 , optical detectors  16 , and semiconductor metal oxide detectors  18 . 
         [0041]    The NIR photodetector  12  can be used to measure gas absorption upon passing a NIR laser through oxygen gas. 
         [0042]    The solid electrolyte detector  14  detects oxygen gas produced during an electrolytic chemical reaction, which can be a spontaneous, or potentiometric reaction  20 , or a non-spontaneous, amperiometric reaction  22 . In a potentiometric reaction  20 , a spontaneous current can be produced by a reaction inside an electrolyte such as Bi 2 O 3  or Gd 2 O 2  doped with CeO 2  or YSZ. In an amperiometric reaction  22 , an external source of current can drive oxygen from one electrode to another through the electrolyte. 
         [0043]    Optical detectors  16  operate on the principle that oxygen quenches luminescence of an organic indicator such as fluoranthane or platinum-octaethylporphyrin dye. 
         [0044]    Semiconductor metal oxide detectors  18  work on the principle that oxidation of a metal oxide semiconductor produces a change in resistance of the semiconducting material. 
         [0045]    Another type of oxygen gas sensor is a magnetic field-based oxygen sensor  24  such as the one shown in  FIG. 3 . The magnetic field-based oxygen sensor  24  relies on the paramagnetic property of oxygen gas. Because oxygen is paramagnetic, it can be influenced by magnetic fields. Furthermore, the presence of oxygen within a medium changes the magnetic susceptibility of the medium. 
         [0046]    A conventional, macroscopic, magnetic field-based oxygen sensor  24  includes a measuring cell  26  in the form of a gas chamber housing a magnet. The magnet has a north pole  28  and a south pole  30  to establish a magnetic field oriented downward along a vertical axis  32 . Oxygen gas enters the measuring cell  26  at a gas inlet  34 , flows along a gas flow axis  36  and exits at a gas outlet  38 . A dumbbell  39  aligned with the gas flow axis  36  can freely rotate about a spindle  40  in response to the flow of oxygen gas as the gas passes through the measuring cell  26 . A plane mirror  41  can be positioned in the center of the dumbbell  39  between the poles of the magnet such that when the dumbbell  39  is deflected by gas flow, the mirror  41  tilts accordingly. External to the measuring cell  26 , a light source  42  generates a source light beam  44  for incidence on the mirror  41 . A conventional source light beam  44  may be, for example, a laser having power in the range of about 1-10 Watts. A reflected light beam  46  can then be detected at a photodetector  48 . Signals from both the light source  42  and the photodetector  48  can be enhanced by an amplifier  50  to produce a measurement readout on an indicating unit  51 . As long as the flow of oxygen gas through the measuring cell  26  remains constant, the reflected light beam  46  sensed by the photodetector  48  will also tend to have a constant intensity and be received at the same angle. 
         [0047]    However, when a change in the magnetic field strength causes the oxygen flow to vary, the angle of the mirror  41  will deflect in response to the change in gas flow. A deflection in the mirror angle will then cause fluctuation in the reflected light beam  46  sensed at the photodetector  48 . 
         [0048]      FIG. 4A  shows a schematic diagram of a Hall circuit  52  for use with a generic Hall effect sensor. The Hall effect describes a tendency for charges in a current-carrying wire to drift to one side of the wire when the wire is placed in an external magnetic field. This drift is due to the Lorentz force, F=qv×B. As positive charges drift in one direction and negative charges drift in the opposite direction, a potential difference is set up across the wire, orthogonal to the direction of current flow. Such a potential difference is well known to those of skill in the art as a Hall potential, or Hall voltage. Hall effect sensors are often used to determine the sign of charge carriers in semiconductor devices, for example, in which both hole currents and electron currents can exist. In the present context, O 2  gas molecules, having a net dipole moment, can behave like free charges in motion, which causes the molecules to drift in a magnetic field, thus producing a Hall voltage. 
         [0049]    The Hall circuit  52  includes a Hall effect sensor  54 , a voltage regulator  56 , and an operational amplifier (op-amp)  58 . A bias voltage V input  can be applied through the voltage regulator  56  at a bias voltage contact  60 . The bias voltage can induce a Hall voltage between the terminals  62  and  64  of the Hall effect sensor  54 . If the Hall voltage is then input to the op-amp  58 , the Hall voltage can be amplified for ease of monitoring at V output . 
         [0050]      FIG. 4B  shows a switching plot  66  of voltage vs. applied magnetic field for the Hall effect sensor  54  used in a magnetic switch application. As the polarity of the applied magnetic field switches through the range of about −150 Gauss to +150 Gauss, a Hall voltage rises linearly from about 0 V (off state) to about 10 V (on state). Because the Hall voltage arises due to the paramagnetic response of the O 2  gas, the O 2  concentration affects the slope  68  of the switching plot  66 . As the O 2  concentration increases, the slope  68  increases accordingly. At about 150 Gauss, the Hall voltage saturates and the slope  68  returns to zero. 
         [0051]      FIG. 5  shows a conventional semiconductor implementation of a Hall effect sensor  54  according to one exemplary embodiment. The Hall effect sensor  70  as shown is a four-terminal device having voltage contacts V 1  ( 72 ), V bias  ( 74 ), and V 2  ( 76 ), and a ground contact ( 78 ). Such voltage contacts are typically made of a metal (e.g., Al). The V 1  voltage contact  72  and the ground contact  78  are coupled to an N+-doped region  80  within an epitaxially-grown surface layer  83  in contact with a crystalline silicon substrate  84 . Within the surface layer  83  are formed P-N junctions  84  and  86 . A downward-directed magnetic field B ( 88 ) can be applied externally as shown. Upon biasing the Hall effect sensor  70  by applying a bias voltage between the V bias  voltage contact( 74 ) and the ground contact ( 78 ), a current I hall  ( 87 ) flows and a voltage V hail  ( 82 ) can be measured between the voltage contacts V 1  ( 72 ) and V 2 ( 76 ). 
         [0052]      FIGS. 6A-6C  show a time sequence of snapshots of a miniature magnetic field-based gas sensor  98 , according to one embodiment used to carry out a method of detecting a paramagnetic gas. Such a miniature magnetic field-based gas sensor  98  that is appropriate for consumer electronics desirably operates at power levels in the range of about 1-10 mW. This power range represents about a 1000× power reduction compared to the large scale system described above with reference to  FIG. 3 . In addition, it is advantageous for the lifetime of such a miniature magnetic field-based gas sensor  98  to exceed that of a conventional sensor, which typically operates for about 1-5 years. The lifetime of the miniature magnetic field-based gas sensor  98  is, in theory, unlimited, because it includes no moving parts and does not consume material in chemical reactions. Thus, the miniature magnetic field-based sensor  98  is more durable than other types of gas sensors. 
         [0053]    The miniature magnetic field-based gas sensor  98  includes a gas sensing cavity  100  positioned in an external downward-directed magnetic field  88 . 
         [0054]    As shown in  FIG. 6A , the gas sensing cavity  100  is empty, prior to introducing a gas sample. In one embodiment, the gas sensing cavity  100  can have the shape of a rhombus having a width of about 50 microns and a height of about 10 microns. The external downward-directed magnetic field  88  can be supplied by an external permanent magnet having a magnetic north pole  104  and a magnetic south pole  106 . The orientation of the external downward-directed magnetic field  88  is indicated by vertical magnetic field lines. The gas sensing cavity  100  can be formed within a glass carrier  108 , for example, by an oxide etching process, such as a wet etch using a pure, concentrated hydrofluoric acid that etches glass at a rate of about 15 μm/minute at 50 C. The glass carrier  108  can then be inverted and bonded to the crystalline silicon substrate  84  in which an integrated Hall effect sensor  70  has been formed. The crystalline silicon substrate  84  can have a thickness of, for example, within the range of about 50 microns-150 microns. The glass carrier  108  can have a thickness within the range of about 700 microns-1 mm. The Hall effect sensor  70  is designed to make a four-point Hall measurement, e.g., two currents and two voltages. In particular, the Hall effect sensor  70  provides leads to measure the current at the second pad  115  and a voltage difference between the first pad sensor contact  114  and the second sensor contact pad  115 . The voltage contacts V 1  ( 72 ) and V 2  ( 76 ) in  FIG. 5  are similar to the sensor contact pads  114  and  115 , respectively. A volt meter  116  can be used to monitor potential differences ΔV (Hall voltage  82 ) between the first sensor contact pad  114  and the second sensor contact pad  115 . Prior to introducing a sample, the volt meter  116  may read, for example, 1 mV. 
         [0055]    The method of detecting oxygen gas using the magnetic field-based gas sensor  98  entails applying a magnetic field to the miniature magnetic field-based gas sensor  98  ( FIG. 6A ), introducing a gas sample ( FIG. 6B ), and measuring a Hall voltage indicative of a presence of oxygen in the gas sample ( FIG. 6C ). As shown in  FIG. 6B , when a gas sample containing oxygen molecules  118  (e.g., air) is introduced into the gas sensing cavity  100 , initially the volt meter  116  still measures 1 mV. This is because the oxygen molecules are still randomly distributed within the sensing cavity  100 . Then, because it has the property of paramagnetism, the oxygen gas responds to the applied magnetic field  88  as shown by the segregation  122  of O 2  molecules in the gas sensing cavity  100 . Some of the O 2  molecules move toward the magnetic north pole  104 , while others move toward the magnetic south pole  106 , according to the effect of the magnetic field  88  on the dipole moments of the oxygen atoms. As the oxygen gas becomes magnetized, the susceptibility decreases the magnetic field intensity within the gas sensing cavity  100 . Segregation  122  of the O 2  molecules in the magnetic field  88  thus results in a decrease in the electric potential difference across the integrated Hall effect sensor  70  as detected by the volt meter  116 . After a short time interval, the volt meter  116  reads 0.8 mV, indicating development of a Hall voltage  82  of 0.2 V across the gas sample, which can be related to an oxygen concentration within the gas sample ( FIG. 6C ). 
         [0056]    The integrated Hall effect sensor  70  used in the present context is highly sensitive, being capable of detecting a momentary magnetic field of a few μTesla. Therefore, it can feature a very fast response time (e.g., milliseconds or microseconds), as compared with a macroscopic conventional oxygen detector (e.g., the one shown in  FIG. 3 ) which is typically capable of sensing magnetic fields of 30-50 mTesla, and has a typical response time of 10 seconds-1 minute. 
         [0057]    The miniature magnetic field-based gas sensor  98  may be configured to operate alongside a control sensor. For example,  FIG. 7  shows a differential micro-structure  126  that senses oxygen gas (O 2 ) by measuring a differential Hall effect. The differential micro-structure  126  includes the miniature magnetic field-based gas sensor  98  and a reference element  130  that share a common micro-channel  141 . The views of the miniature magnetic field-based gas sensor  98  shown in  FIGS. 6A-6C  are cross-sectional views taken along cut line A-A′ of the present view shown in  FIG. 7 . The miniature magnetic field-based gas sensor  98  allows monitoring of O 2  gas in a sample of ambient air that enters the micro-channel  141  of the differential micro-structure  126  through one or more openings  136 ,  138 , and  140 . The openings  136 ,  138 , and  140  are sized and dimensioned to allow a constant volume of a few microliters of air to be maintained within the micro-channel  141  which spans both of the sensing elements. The openings  136 ,  138 , and  140  can be made, for example, by drilling. Thus, the reference element  130  is in fluid communication with the gas sensing cavity  100  (i.e., the reference element contains the same gas as the gas sensing cavity  100 ). A magnetic field  88  is then applied to the miniature magnetic field-based gas sensor  98 , in a direction orthogonal to the plane in which the sensors lie, as indicated by the “X.” Meanwhile, no magnetic field is applied to the reference element  130 . In response to the applied magnetic field  88 , O 2  molecules in the miniature magnetic field-based gas sensor  98  tend to segregate, giving rise to a lateral charge separation, whereas O 2  segregation does not occur in the reference element  130 . The charge separation causes an electric potential difference to form between the two poles V+ and V−. This potential difference is then measured as the voltage V sensor , for comparison with a reference voltage V reference . The voltage difference 
         [0000]      Δ V   out   =V   sensor   −V   reference    (1)
 
         [0000]    represents a change in magnetization of the oxygen gas in response to the applied magnetic field. In general, magnetization M varies with the applied magnetic field  88  according to the relationship 
         [0000]      M=xB,   (2)
 
         [0000]    in which x is proportional to the volume susceptibility of the O 2  gas, the susceptibility being a material constant. The voltage difference ΔV out    144  is thus a differential Hall voltage. 
         [0058]      FIG. 8  shows a family of curves  144  of measured values of the differential Hall voltage ΔV out  ( 144 ) as a function of the external, downward-directed magnetic field strength B ( 88 ) for three gas samples, each containing a different concentration of O 2 . As expected from equation (2), each of the curves  144  is linear. A gas sample containing 1% oxygen produces a linear curve  146 ; a sample containing 2% oxygen produces a linear curve  148 , and a sample containing 3% oxygen produces a linear curve  150 . As the gas concentration increases, the external, downward-directed magnetic field  88  is less effective in segregating the O 2  molecules, so the differential voltage is lower for the same applied magnetic field strength. 
         [0059]    The side view of the miniature magnetic field-based gas sensor  98  is reproduced in  FIG. 9A , for comparison with a corresponding side view of an alternative oxygen sensor  156  shown in  FIG. 9B . The alternative oxygen sensor  156  features an integrated thin film permanent magnet  158  in place of the external permanent magnetic poles  104 ,  106 . The integrated thin film permanent magnet  158  may be, for example, an Iron-Cobalt-Nickel (FeCoNi) magnet having a thickness of about 1 micron. Such an integrated thin film permanent magnet  158  can be deposited and patterned after etching the gas sensing cavity  100  from the glass carrier  108 . After the glass carrier  108  is placed over the crystalline silicon substrate  84 , the integrated thin film permanent magnet  158  extends downward toward the center of the gas sensing cavity  100 . 
         [0060]    The top plan view of the miniature magnetic field-based gas sensor  98  shown in  FIG. 7  is reproduced in  FIG. 10A , for comparison with a corresponding top plan view of the alternative oxygen sensor  156  shown in  FIG. 10B . The alternative oxygen sensor  156  also includes the micro-channel  141  including the gas inlet opening  136  and the gas outlet opening  138 . The sensors  98  and  156  function according to the same principles and share substantially the same structure, except for the geometry of the magnets. Thus, the alternative oxygen sensor  156  is a substitute for the miniature magnetic field-based gas sensor  98 , with the exception that the alternative oxygen sensor  156  includes the thin film permanent magnet  158 . It is noted that the thin film permanent magnet  158  has a smaller size than the external magnet characterized by magnetic poles  104 ,  106 . Whereas the external downward-directed magnetic field  88  is represented by a large circumscribed “X” in  FIG. 10A , the corresponding thin film magnetic field  162  produced by the thin film permanent magnet  158  shown in  FIG. 10B  is represented by a small rectangle. However, the size of the magnet itself does not necessarily dictate the strength of the magnetic field produced thereby. Field lines associated with the external downward-directed magnetic field  88  extend farther out along the micro-channel  141 , whereas field lines produced by the thin film permanent magnet  158  may tend to be more concentrated in the region of the gas sensing cavity  100 , directly above the sensor contact pads  114  and  115 . 
         [0061]      FIGS. 11A-11C  show a time sequence of snapshots of the alternative oxygen sensor  156 . The alternative oxygen sensor  156  operates at power levels similar to those used for the miniature magnetic field-based gas sensor  98 . Also, the lifetime of the alternative oxygen sensor  156  is comparable to the extended lifetime of the miniature magnetic field-based gas sensor  98 . The alternative oxygen sensor  156  includes the gas sensing cavity  100  positioned in the thin film magnetic field  162  produced by the thin film permanent magnet  158 . As shown in  FIG. 11A , the gas sensing cavity  100  is empty prior to introducing a gas sample. The orientation of the thin film magnetic field  162  is indicated by vertical magnetic field lines. As described above with reference to  FIGS. 6A-6C , the volt meter  116  can be used to monitor potential differences ΔV (Hall voltage  82 ) between the first sensor contact pad  114  and the second sensor contact pad  115 . Prior to introducing a sample, the volt meter  116  may read, for example, 1 mV. 
         [0062]    As shown in  FIG. 11B , when a gas sample containing oxygen molecules  164  (e.g., air) is introduced into the gas sensing cavity  100 , initially the volt meter  116  still measures 1 mV. This is because the oxygen molecules are still distributed randomly within the gas sensing cavity  100 . Then, because it has the property of paramagnetism, the oxygen gas responds to the applied thin film magnetic field  162  as shown by the segregation  166  of O 2  molecules in the gas sensing cavity  100  ( FIG. 11C ). Some of the O 2  molecules move toward the magnetic north pole, while others move toward the magnetic south pole, according to the Hall effect. The segregation  166  of the O 2  molecules in the thin film magnetic field  162  again tends to decrease the magnetic field strength, and in turn, the electric potential difference, across the integrated Hall effect sensor  70  as detected by the volt meter  116 . After a short time interval the volt meter  116  reads 0.8 mV, indicating development of a Hall voltage  82  of 0.2 V across the gas sample which can be related to an oxygen concentration within the gas sample ( FIG. 11C ). Like that in the miniature magnetic field-based gas sensor  98 , the integrated Hall effect sensor  70  in the alternative oxygen sensor  156  is capable of detecting a momentary magnetic field in the range of a few μTesla. Therefore, the alternative oxygen sensor  156  can also feature a very fast response time (e.g., milliseconds or microseconds), as compared with a macroscopic conventional oxygen detector (e.g., that shown in  FIG. 3 ). 
         [0063]    In one embodiment, some or all of the logic circuits that form the op-amp  58  and other logic devices of the sensing circuits are formed in the same substrate  84  that contains the contact pads  114  and  115 . This reduces the distance that a signal needs to travel from the analog sense location  100  to the electrical detection and measurement circuit, which includes the op-amp  58  and other circuits. It also reduces the amount of noise that can enter the signal prior to being placed in digital form. In such embodiments, the substrate  84  is a monocrystalline substrate in which transistors and logic circuits can be formed. In other embodiments, the substrate  84  is a polycrystalline or amorphous substrate. Thus, in one embodiment, the substrate  84  can be polycrystalline silicon, amorphous silicon, quartz, sapphire, or other substrate material. 
         [0064]      FIG. 12  shows an exemplary sequence of steps in a fabrication process  170  for making a miniature magnetic field-based oxygen sensor. 
         [0065]    At  172  a miniature Hall effect sensor (e.g., a sensor resembling the conventional semiconductor-based Hall effect sensor  70 ) can be defined on the crystalline silicon substrate  84  according to the cross section shown in  FIG. 5 . 
         [0066]    At  174 , the top metal layer of the Hall effect sensor  70  can be patterned to provide electrical sensor contact pads  114  and  115  (V 1  and V 2  contacts, respectively), at which a Hall voltage can be measured. 
         [0067]    At  176 , a metal film (e.g., a chromium-gold (Cr—Au) film) can be deposited onto the amorphous (e.g., glass) substrate  108  and patterned to form a micro-channel mask. The chromium-gold film may be in the form of a bi-layer wherein the chromium thickness is about  250  nm and the gold thickness is about 5000 nm. 
         [0068]    At  178 , the amorphous substrate  108  can be etched through the micro-channel mask to form the micro-channel  141  so as to have a channel depth of about 20 μm. 
         [0069]    Once the micro-channel  141  is formed, at  180 , the openings  136 ,  138 , and  140  can be drilled into the amorphous substrate  108  to provide access for ambient air to enter and be maintained at a constant volume in the micro-channel  141 . A flow of air is not needed to perform the Hall effect measurements. The area of the openings  136 ,  138 ,  140  can be about (100 μm) 2 . 
         [0070]    Optionally, at  181 , a ferromagnetic film (e.g., an FeCoNi film) can be deposited and patterned on a surface of the micro-channel  141  to form the integrated thin film permanent magnet  158 . The thin film permanent magnet  158  is sized and dimensioned to fit, or to define, the gas-sensing cavity  100  for the alternative oxygen sensor  156 . 
         [0071]    At  182 , the amorphous substrate  108  can be bonded to the crystalline silicon substrate  84  using anodic bonding, metal eutectic bonding, or an adhesive bonding technique. Examples of adhesives that may be used in bonding such substrates include epoxy, or a polyimide film. 
         [0072]    At  184 , the bonded substrates  108  and  84  can be diced into individual sensor chips. At  186 , the sensor chips can be assembled into an open package. 
         [0073]    The open package may support one or more sensor chips that are stacked with, or placed adjacent to, other sensor chips and/or integrated circuit chips carrying signal processing circuitry, microprocessors, electronic memory, and the like. During a packaging process, the chips may be electrically coupled to one another by wire bonding. 
         [0074]    The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments 
         [0075]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.