Abstract:
A sensor chip exposed to a flowing medium is provided with an additional heater situated upstream and at a definite distance from the sensor area, resulting in the impurities in the flowing medium being deposited in the area of the additional heater, and in the impurities being unable to reach the sensor area.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This case is a 371 of PCT/DE02/00894 filed on Mar. 13, 2002. 

   FIELD OF THE INVENTION 
   The present invention is directed to a sensor chip, use of an additional heater on a sensor chip, and a method of preventing contamination on a sensor chip. 
   BACKGROUND INFORMATION 
   German Patent Application 196 01 791 describes a sensor chip having a sensor area composed of a frame element, a recess, and a membrane, for example. An unwanted influence on the measuring signal of the sensor chip in the sensor area may occur repeatedly due to contamination, e.g., oil, to which the sensor chip is exposed. Contamination with oil in the sensor area or in the immediate area around the sensor area alters the thermal conductivity at the surface of the sensor chip and thus affects the measuring signal. In addition, oil deposited on the sensor chip forms an adhesive for particles contained in a flowing medium. These trapped particles further increase the unfavorable effect. 
   U.S. Pat. No. 5,705,745 describes a sensor chip having a membrane on which are arranged temperature resistors and heating resistors, the membrane being surrounded by a thermally conductive element, which may also be U-shaped. The thermally conductive element is not heated. The thermally conductive element is also situated at least partially in the area of the membrane. 
   U.S. Pat. No. 4,888,988 describes a sensor chip having a membrane, a metallic conductor being situated around the is the common grounded neutral conductor of the measuring arrangement on the sensor chip. The cross section of this grounded neutral conductor has even been increased selectively to prevent an increase in temperature. An elevated temperature of the grounded neutral conductor would also have an extremely deleterious effect on the measurement according to this method. 
   German Patent Application 198 01 484 describes a sensor chip having a membrane, electric conductors being situated around the membrane with an electric current flowing through them. These conductors are temperature sensors which are used for the measurement method and/or the measurement procedure. 
   German Patent Application 2 900 210 and U.S. Pat. No. 4,294,114 describe a sensor chip having a temperature-dependent resistor on a carrier, another resistor directly adjacent to the temperature-dependent resistor also being applied to the carrier. 
   German Patent Application 4 219 454 and U.S. Pat. No. 5,404,753 describe a sensor chip having a reference temperature sensor at a distance from a sensor area. 
   German Patent Application 3 135 793 and U.S. Pat. No. 4,468,963 describe a sensor chip having another resistor upstream and/or downstream from the sensor resistor, but the additional resistor influences the measuring signal. 
   SUMMARY 
   The sensor chip according to the present invention, the use of an additional heater on a sensor chip according to the present invention, and the method according to the present invention for preventing contamination of a sensor chip, may provide that contamination of the sensor area of the sensor chip is reduced or prevented in a simple way. 
   An additional heater situated at a distance of up to 1 mm from the membrane may be used, so that the precipitates which are intentionally formed there are far enough away from the sensor area and are unable to influence the measurement response of the sensor area. 
   The additional heater may be in a U shape in a manner enclosing the sensor area. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates an example embodiment of a sensor chip. 
       FIG. 2   a  illustrates a first example embodiment of a sensor chip according to the present invention. 
       FIG. 2   b  illustrates a second example embodiment of a sensor chip according to the present invention. 
       FIG. 2   c  illustrates a third example embodiment of a sensor chip according to the present invention. 
       FIG. 3   a  illustrates an example embodiment of a sensor chip according to the present invention. 
       FIG. 3   b  illustrates an example embodiment of a control circuit according to the present invention. 
       FIG. 4  shows a temperature profile of an additional heater and a sensor area of an example embodiment of a sensor chip according to the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows a conventional sensor chip which is improved according to the present invention according to the descriptions of  FIGS. 2   a  through  2   c . The production method and the use of such a sensor chip are described in detail in German Patent Application 196 01 791, which is herewith explicitly to be included as part of the present disclosure. 
   The sensor chip has a frame element  3  made of silicon for example. Frame element  3  has a recess  5 . A dielectric layer  21 , e.g., made of SiO 2 ′ for example, may be applied to the frame element. Layer  21  may extend over entire frame element  3  or only over an area of recess  5 . This area forms a membrane area  7  which partially or entirely delimits recess  5  on one side. 
   At least one, e.g., three metal strips  19  are applied on the side of membrane area  7  facing away from recess  5 . Metal strips  19  form electric heaters and/or measuring shunts, for example, forming a sensor area  17  together with membrane area  7 . Sensor area  17  may be covered with a protective layer  23 . Protective layer  23  may also extend only over metal strips  19 . 
   Membrane area  7  is then formed in part by dielectric layer  21  producing a measuring signal, a membrane  33  and in part by protective layer  21 . The sensor chip has a surface  27  which is in direct contact with a flowing medium. 
     FIG. 2   a  shows a top view of a first example embodiment of a sensor chip  1  designed according to the present invention. Sensor chip  1  has a sensor area  17  having a length l across a main direction of flow  42 . For example, metal strips  19  which form at least one electric heating resistor  35  and at least one temperature sensor  37 , for example, are situated in sensor area  17 . Temperature sensor  37  is also an electric resistor, for example. In this case there will be one heating resistor  35  and two temperature sensors  37 . Metal strips  19  may be mostly situated in sensor area  17  and may be used for a measurement method for determining at least one parameter, e.g., the temperature and flow rate of the flowing medium. Sensor area  17  is therefore connected to a control and regulating circuit. Sensor area  17  may be formed by membrane  33  described above, for example. Sensor chip  1  is situated in a flowing medium for determination of at least one parameter, the flowing medium flowing in the main direction of flow  42 , an or over sensor chip  1 , i.e., surface  27 . The flowing medium may contain impurities which may result in contamination of sensor chip  1 . These include, for example, salts dissolved in water or oil. To prevent these impurities from being deposited in the area of sensor area  17 , at least partially an additional heater  39  may be situated upstream from sensor area  17 , for example, connected to a current source, and heated by its ohmic resistance. Additional heater  39  is situated at a defined distance, e.g., up to 1 mm away from sensor area  17 . 
   No control circuit is necessary to regulate the temperature of additional heater  39 . An amperage determined by the design, i.e., by the cross section is sufficient. Additional heater  39  is not used for a measurement method for determining a parameter of the flowing medium, i.e., it is not a component of this measurement zone. 
   Additional heater  39  may be in the form of a straight line, for example, which extends across, e.g., perpendicular to, the main direction of flow  42 , e.g., extending beyond a length l of sensor area  17 . Additional heater  39  may also have a spiral shape. Due to additional heater  39 , contamination of sensor chip  1  may occur in the area of additional heater  39 , but at a definite distance away from sensor area  17  so that the measurement response of sensor area  17  is not affected. This contamination is thus displaced from sensor area  7  into the area around additional heater  39 . 
   The temperature of additional heater  39  may be set so that there is a sharp temperature transition in the area of additional heater  39 , so that thermal gradient eddies are produced, more or less filtering the liquid or the oil out of the flowing medium, i.e., the heavier components of the flowing medium are deposited on surface  27  in the area of additional heater  39  but not in sensor area  17 . 
     FIG. 2   b  shows a top view of another example embodiment of sensor chip  1  according to the present invention. In contrast with  FIG. 2   a , additional heater  39  is U shaped. The U shape of additional heater  39  may be situated on sensor chip  1  at a definite distance away upstream from sensor area  17 , the two legs of the U shape running across main direction of flow  42 . 
     FIG. 2   c  shows a top view of another example embodiment of a sensor chip  1  according to the present invention. Additional heater  39  may have a U shape which at least partially encloses sensor area  17 . Additional heater  39  runs on downstream and upstream sides, for example, definitely at a distance from sensor area  17  and on an end face of membrane  33 . 
   Additional heater  39  may be designed, for example, so that it has a length greater than that of sensor area  17 , for example, at least upstream or downstream from sensor area  17 . Therefore, sensor area  17  may be protected from contamination over its entire length l. 
   Resistors  35 ,  37  and/or additional heater  39  may be designed as printed conductors. 
   Sensor chip  1  is designed in the form of a chip, for example and has surface  27  past which the flowing medium flows. Sensor area  17  and additional heater  39  are situated together on surface  27 . 
     FIG. 3   a  shows an example embodiment of a sensor chip  1  designed according to the present invention, having a sensor area  17  and a first control circuit  54  which is electrically connected to sensor area  17  by electric conductors  51 , e.g., bond wires. First control circuit  54  has a first power source  45 , e.g., a current or voltage source, or it is connected electrically to such a source by which at least one heating resistor  35  or at least one temperature sensor  37  is heated electrically in sensor area  17 . 
   Additional heater  39  is connected to a separate second power source  48 , for example, via electric conductors  51 . There is no electric connection between first control circuit  54  and second power source  48 . First control circuit  54  thus supplies a measuring signal, e.g., for an engine controller which is independent of operation of additional heater  39 , i.e., the operation of additional heater  39  has no effect on the measuring signal. First power source  45  may also heat additional heater  39 , e.g., via a voltage splitter, but the control signal of first power source  45  to additional heater  39  may still be independent of the measurement method or signals to sensor area  17 . 
   Sensor chip  1  supplies a measuring signal, e.g., for regulating an internal combustion-engine. Additional heater  39 , for example, may be heated only when the engine is not in operation, because after shutdown of the engine may be when the most frequent contamination of sensor chip  1  occurs due to backflow, e.g., from crankcase venting, containing contaminants such as oil. First control circuit  54  may deliver the signal for the heating operation of additional heater  39 , for example, by closing a switch  60 , for example, so that second power source  48  heats additional heater  39 . 
   This control signal for heating additional heater  39  when the engine is not in operation may also be supplied by a second control circuit  57 . Second control circuit  57  is the engine regulating unit, for example ( FIG. 3   b ). 
     FIG. 4  shows a temperature profile of an example embodiment of additional heater  39  and sensor area  17 .  FIG. 4  shows an X/Y diagram, a length in main direction of flow  42  being plotted on the X axis and a temperature on the surface of sensor chip  1  being plotted on the Y axis. 
   Additional heater  39  is located, for example, upstream from sensor area  17 . Between additional heater  39  and sensor area  17  there is a distance which is different from zero. For example, the resistors in sensor area  17  generate a trapezoidal temperature curve having a maximum temperature T M . 
   Additional heater  39  has a maximum temperature T Z  which varies according to a parabolic curve, for example, which is equal to or greater than temperature T M . 
   Arrows  62  show the flow pattern of the medium near surface  27 . Additional heater  39  creates a more or less abrupt increase in temperature at surface  27 , i.e., a thermal gradient which is large and differs from zero. Oncoming particles near surface  27  are more or less sucked by a partial vacuum to surface  27  upstream from or at the initial area of additional heater  39 , and then rise upward in the area of the additional heater, i.e., removing themselves from surface  27 . Due to this flow pattern, thermal gradient eddies  65  are created in the area of additional heater  39 . Particles of dirt or oil therefore adhere to surface  27  of sensor chip  1  in the area of additional heater  39 , so that the flowing medium is cleaned in the area near the surface, and sensor area  17  has little or no contamination.