Patent Publication Number: US-11391611-B2

Title: Thermal airflow sensor

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/979,301, filed May 14, 2018 which is a continuation of U.S. application Ser. No. 15/495,268, filed Apr. 24, 2017 which is a continuation of U.S. application Ser. No. 14/410,713, filed Dec. 23, 2014, which is a 371 of International Application No. PCT/JP2013/065912, filed Jun. 10, 2013, which claims priority from Japanese Patent Application No. 2012-146286, filed Jun. 29, 2012, the disclosures of which are expressly incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a sensor that detects physical quantities. More particularly, the invention relates to a thermal airflow sensor. 
     BACKGROUND ART 
     Thermal airflow sensors have conventionally been a mainstream airflow sensor that is installed in the intake air passage of internal combustion engines, such as those of automobiles, to measure intake air volume since the thermal airflow sensors are capable of directly detecting amount of air. 
     Recently, there has been developed an airflow sensor formed by having resistors and insulating layer films deposited on a silicon substrate by use of semiconductor micromachining technology, part of the silicon substrate being removed thereafter by a solvent represented by KOH to form a thin-wall portion. This airflow sensor is drawing attention because it has high-speed responsiveness and is capable of detecting counter flows thanks to its quick response. In recent years, for the purpose of reducing the number of components constituting the substrate portion (printed substrate, silicon substrate, etc.), study has been underway to form a structure in which this airflow sensor is mounted on a lead frame of which the periphery is molded in resin. 
     PRIOR ART LITERATURE 
     Patent Document 
     
         
         Patent Document 1: Japanese Patent No. 3610484 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     An existing thermal airflow sensor described in Patent Document 1 is an invention in which the surface of a flow sensor element is provided with a protective film made of an organic material for the purpose of improving the reliability of the thin-wall portion formed by removing part of the silicon substrate from its back side. According to Patent Document 1, the insulating film of the thin-wall portion is given enhanced resistance to dust. With this invention, however, there is room for consideration in partially exposing an area that includes the thin-wall portion in a structure in which the flow sensor element is adhesively attached to a member such as the lead frame, with the periphery of the flow sensor element sealed with resin. 
     When resin molding is performed for partial exposure, it is general practice to press a metal mold, an insertion die or the like onto the periphery of the thin-wall portion over a semiconductor detection element during molding so that a mold resin material will not be formed in the exposed portion. A principal method for pressing the insertion die involves controlling the amount of movement of the insertion die. Where mass production is considered, the amount of movement to be set is always constant; the amount of movement is left unadjusted from one product to another. At this point, if the pressing force on the insertion die is not sufficient, the mold resin could flow into the exposed portion. To avoid this eventuality requires pressing the insertion die toward the semiconductor device with a certain level of force. If the pressing force is excessive, the semiconductor device can be deformed. Thus where the area including the thin-wall portion is to be partially exposed when sealed with resin, the force with which to press the insertion die has a margin to certain extent. 
     Also, there are variations in film thickness as well as in adhesive thickness over the detection element from one product to another. As a result, the height of the semiconductor device mounted on the lead frame varies. It follows that that the force applied from the insertion die or the contact distance thereto varies in each product. This further reduces the permissible range of the pressing force toward the insertion die, leading to a decline in throughput yield. 
     An object of the present invention is to improve the reliability of a product in which a semiconductor device is partially exposed when sealed with resin. 
     Means for Solving the Problem 
     In achieving the above object and according to the present invention, there is provided a thermal airflow sensor including: a semiconductor substrate having a thin-wall portion, a heating resistor provided over the thin-wall portion, and resistance temperature detectors installed upstream and downstream of the heating resistor; a protective film provided over the semiconductor substrate; and a resin that seals the semiconductor substrate, the resin further including an exposure portion for partially exposing an area including the thin-wall portion. The protective film is provided in a manner seamlessly enclosing the heating resistor, the protective film having an outer peripheral edge located outside the thin-wall portion and over the exposure portion. 
     Effect of the Invention 
     The present invention improves the reliability of a product in which a semiconductor device is partially exposed when sealed with resin. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a set of structural views showing a sensor element yet to be molded as a first embodiment of the present invention, where  FIG. 1( a )  is a lateral sectional view and  FIG. 1( b )  is an overhead topographic view. 
         FIG. 2  is a set of structural views showing the sensor element molded as the first embodiment, where  FIG. 2( a )  is a lateral sectional view and  FIG. 2( b )  is an overhead topographic view. 
         FIG. 3  is a schematic explanatory view showing how molding is performed on the first embodiment. 
         FIG. 4  is a schematic explanatory view showing how the mold resin flows out in the first embodiment. 
         FIG. 5  is a set of structural views of a sensor element yet to be molded as a second and a third embodiment of the present invention, where  FIG. 5( a )  is a lateral sectional view and  FIG. 5( b )  is an overhead topographic view. 
         FIG. 6  is a set of structural views of the sensor element molded as the second embodiment, where  FIG. 6( a )  is a lateral sectional view and  FIG. 6( b )  is an overhead topographic view. 
         FIG. 7  is a schematic explanatory view showing how molding is performed on the second embodiment. 
         FIG. 8  is a schematic explanatory view showing how the mold resin flows out in the second embodiment. 
         FIG. 9  is an explanatory view of a slit. 
         FIG. 10  is a set of structural views showing the sensor element molded as the third embodiment, where  FIG. 10( a )  is a lateral sectional view and  FIG. 10( b )  is an overhead topographic view. 
         FIG. 11  is a schematic explanatory view showing how molding is performed on the third embodiment. 
         FIG. 12  is a schematic explanatory view showing how the mold resin flows out in the third embodiment. 
         FIG. 13  is a structural view showing a sensor element yet to be molded as a fourth embodiment of the present invention. 
         FIG. 14  is a structural view showing a sensor element yet to be molded as a fifth embodiment of the present invention. 
         FIG. 15  is a structural view showing a thermal airflow sensor according to the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The thermal airflow sensor according to the present invention will now be explained with reference to  FIG. 15 . 
     The thermal airflow sensor of the present invention includes a housing  3  and a semiconductor package  2  installed inside an intake pipe  5  that feeds intake air  1  to an automobile internal combustion engine (not shown). 
     The housing  3  includes a connector terminal  8  coupled electrically to the semiconductor package  2 , a flange portion  4  that fixes the housing  3  to the intake pipe  5 , and an auxiliary passage  6  that admits part of the intake air  1 . 
     The semiconductor package  2  is formed by having a lead frame  10 , a semiconductor substrate  20 , circuit elements, and a temperature sensor sealed integrally with a mold resin  60 . The semiconductor package  2  also has a partially exposed area (not covered with the mold resin  60 ) so as to expose a flow rate detection portion  7  to the intake air. The flow rate detection portion  7  is installed inside the auxiliary passage  6  and calculates the flow rate of the intake air  1  from the flow rate of a fluid flowing through the auxiliary passage  6 . 
     The first embodiment of the present invention will now be explained with reference to  FIGS. 1 through 4 . 
     The structural views of the sensor element constituting the first embodiment of this invention will be explained below with reference to  FIGS. 1 and 2 . Here,  FIG. 1  is a set of structural views of the sensor element yet to be molded as the first embodiment, and  FIG. 2  is a set of structural views of the sensor element molded as the first embodiment. 
     As shown in  FIG. 1 , the thermal airflow sensor has a semiconductor substrate  20  typically made of silicon and deposited with insulating film and resistor layers. A thin-wall portion  25  is formed by removing part of the back side of the semiconductor substrate  20  typically by use of potassium hydroxide (KOH). A heating resistor  21 , an upstream resistance temperature detector  22 , and a downstream resistance temperature detector  23  are formed over the thin-wall portion  25 . The temperature of the heating resistor  21  is subjected to feedback control so that the temperature of the heating resistor  21  remains higher than that of the intake air  1  by predetermined degrees. The flow rate of the intake air  1  is measured from information about the difference between the temperature measured by the upstream resistance temperature detector  22  and the temperature measured by the downstream resistance temperature detector  23 . An organic protective film  30  typically made of polyimide is formed over the surface of the thermal airflow sensor. The organic protective film  30  is coated once, uniformly, all over the sensor surface by use of a coating machine such as a spinner. Thereafter, the coat is partially removed by etching through patterning technology to form a stagger between the semiconductor substrate  20  and the organic protective film  30 . The organic protective film  30  is shaped seamlessly to enclose the heating resistor  21 . Because the flow rate of the intake air is measured with the heating resistor  21 , upstream resistance temperature detector  22  and downstream resistance temperature detector  23 , these three detectors need to be exposed to the intake air, so that they are not covered with the organic protective film. Also, AI wiring  40  is formed over the surface of the thermal airflow sensor. The AI wiring  40  is electrically coupled to the lead frame  10  by means of a bonding wire  50  such as a gold wire. The semiconductor substrate  20  is fixed to the lead frame  10  with adhesive or the like. 
     As shown in  FIG. 2 , the semiconductor substrate  20  and lead frame  10  are sealed with the mold resin  60 . Here, because the heating resistor  21 , upstream resistance temperature detector  22  and downstream resistance temperature detector  23  need to be exposed to a medium to be measured for flow rate detection, there is provided a structure in which an area including the flow rate detection portion  7  is partially exposed from the mold resin  60  (not covered therewith). Furthermore, the peripheral edge of the organic protective film  30  enclosing the heating resistor  21  is located outside the thin-wall portion  25  in a manner partially exposing the organic protective film  30 . With this structure, even if the resin leaks out of between the surface of the thermal airflow sensor and the insertion die  83  during molding, the leak is stemmed by the organic protective film  30  so that the resin will not reach the thin-wall portion  20 . 
     Molding on the first embodiment will be explained below with reference to  FIGS. 3 and 4 .  FIG. 3  is a schematic explanatory view showing how molding is performed on the first embodiment, and  FIG. 4  is a schematic explanatory view showing how the mold resin flows out in the first embodiment. 
     As shown in  FIG. 3 , a semiconductor package of a partially exposed structure is formed with a lower metal mold  80 , an upper metal mold  81 , and the insertion die  83  provided to be inserted into the upper metal die  81 . The thermal airflow sensor having the semiconductor substrate  20  mounted on the lead frame  10  is sandwiched between the lower metal mold  80  and the upper metal mold  81 . The insertion die  83  is pressed toward the thin-wall portion  25  and other parts to be partially exposed so that these locations will not be covered with the mold resin  60 . The resin is then allowed to flow in through an insertion opening  82  to produce the semiconductor package of the partially exposed structure. The insertion opening  82  may be provided through either the lower metal mold  80  or the upper metal mold  81 . When the pressing portion of the insertion die  83  is pressed toward the substrate surface, the area toward which the insertion die  83  is pressed is not sealed with the resin and thus exposed partially. However, since the thin-wall portion  25  is thinner than the other portions, pressing the insertion die  83  directly toward the thin-wall portion  25  can deform the thin-wall portion  25 , resulting in flow rate detection errors. Thus the insertion die  81  is structured to have a concave part on its pressing portion so that the thin-wall portion  25  will be fitted into the concave part. The pressing portion formed on the peripheral edge of the concave part is pressed toward the substrate surface so that the insertion die  83  will not come into direct contact with the thin-wall portion  25  upon sealing with the resin. This prevents the load with which to press the insertion die  83  from being applied to the thin-wall portion  25 , so that the thin-wall portion is prevented from getting deformed when the area including the thin-wall portion is partially exposed during sealing with the resin. 
     As shown in  FIG. 4 , if the load with which to press the insertion die  83  is insufficient, a gap may be formed between the surface of the thermal airflow sensor and the insertion die. If the resin is poured in this state, the resin may flow out through the gap  61  between the insertion die  83  and the surface of the thermal airflow sensor. However, the first embodiment of the present invention offers a structure in which the organic protective film  30  encloses the heating resistor  21  and is located in an area partially exposed from the mold resin  60 . In this structure, the organic protective film  30  stems the resin  60  that may leak out through the gap  61 , so that the leaking resin is controlled from reaching the thin-wall portion  25 . Even if the force with which to press the insertion die  83  is insufficient and even if a product is manufactured in which the mold resin  60  leaks to the semiconductor device portion, the performance specification of the product are met as long as the leak does not reach the thin-wall portion  25  that is the sensing area. Thus according to the first embodiment of the present invention, the reliability of the thermal airflow sensor is ensured even where insufficient load on the insertion die  83  results in resin leakage. 
     Here, consider the case where the insertion die  83  is pressed under movement control. Since there are variations in the height of the surface of the semiconductor substrate  20  from one product to another, a semiconductor substrate  20  with a higher height than usual is subject to greater load than usual. Too much load can deform the sensor element. On the other hand, a semiconductor substrate with a lower height than usual forms the gap  61  between the insertion die  83  and the surface of the thermal airflow sensor, and the resin may leak through the gap  61 . According to the first embodiment of the present invention, the reliability of the thermal airflow sensor is ensured even when the load on the insertion die  83  is insufficient. This means that during mass production, the manufacturing margin may be increased in a manner favoring lower load on the insertion die  83 . That in turn improves throughput yield. 
     Moreover, since the thin-wall portion  25  is made of an inorganic material and formed thin and fragile in order to boost thermal insulation characteristics, the thin-wall portion  25  needs to have its strength improved against the impact of dust. In particular, the peripheral edge of the thin-wall portion  25  is more vulnerable to the impact of dust than the other portions. Thus an inner peripheral edge of the organic protective film  30  is positioned at the thin-wall portion  25  as shown in  FIG. 1  so that the peripheral edge of the thin-wall portion  25  is covered with the organic protective film  20  that absorbs the shock of the impact of dust. With this structure, the strength of the thin-wall portion  25  against the impact of the dust included in the intake air is raised. The thermal airflow sensor consequently improves its contamination resistance and higher reliability in its implementation. 
     The second embodiment of the present invention will now be explained with reference to  FIGS. 5 through 9 . The same structures as those of the first embodiment will not be discussed further. 
     The structural views of the sensor element constituting the second embodiment of this invention will be explained below with reference to  FIGS. 5 and 6 . Here,  FIG. 5  is a set of structural views of the sensor element yet to be molded as the second embodiment, and  FIG. 6  is a set of structural views of the sensor element molded as the second embodiment. 
     As shown in  FIG. 5 , the organic protective film  30  provided over the semiconductor substrate  20  has an exposure portion for partially exposing the thin-wall portion  25  so that the heating element  21 , upstream resistance temperature detector  22 , and downstream resistance temperature detector  23  are exposed to the medium to be measured, the organic protective film  30  also having a slit  35  formed in a manner enclosing the thin-wall portion  25 . The slit  35  seamlessly encloses the thin-wall portion  25  so that even if resin leakage occurs during resin molding, the slit  35  traps the mold resin  60  and prevents it from flowing to the thin-wall portion  25 . The organic protective film  30  is provided to protect the rim of the thin-wall portion  25 , which boosts the strength of the thin-wall portion  25  against the impact of the dust included in the intake air. The slit  35  partially exposes the semiconductor substrate  20  from the organic protective film  30 , thus forming a stagger between the exposed surface of the semiconductor substrate  20  and the organic protective film  30 . Preferably, there should be provided a structure in which the AI wiring  40  is covered with the organic protective film  30  for protection against corrosive components such as water. 
     And as shown in  FIG. 6 , the semiconductor substrate  20  and lead frame  10  are sealed with the mold resin  60  in a manner partially exposing the entire inner peripheral edge and the outer peripheral edge of the slit  35  from the mold resin  60 . With the entire inner peripheral edge of the slit  35  located in an area partially exposed from the mold resin, even if the mold resin  60  leaks out through the gap  61  as shown in  FIG. 8 , the slit  35  traps the mold resin  60  and prevents it from reaching the thin-wall portion  25 . 
     Molding on the second embodiment will be explained below with reference to  FIGS. 7 and 8 .  FIG. 7  is a schematic explanatory view showing how molding is performed on the second embodiment, and  FIG. 8  is a schematic explanatory view showing how the mold resin flows out in the second embodiment. 
     As shown in  FIG. 7 , when the semiconductor package of a partially exposed structure of the second embodiment is produced, the pressing portion of the insertion die  83  is pressed toward the organic protective film  30 . Because the organic protective film  30  acts as a buffer material that absorbs the stress propagated to the thin-wall portion  25 , deformation of the thin-wall portion  25  is restrained during molding. Thus according to the second embodiment of the present invention, detection errors attributable to the deformation of the thin-wall portion  25  are restrained, and hence that the reliability of the thermal airflow sensor is improved. 
     Further advantages of providing the slit  35  in the organic protective film  30  will be explained below with reference to  FIG. 9 . 
     In a structure where the mold resin  60  is applied to the thermal airflow sensor with the organic protective film  30  interposed therebetween, the organic protective film  30  is stressed due to resin contraction after molding. Where the organic protective film  30  is shaped to communicate with the thin-wall portion edge, the stress caused by resin contraction of the mold resin  60  may reach the edge of the thin-wall portion  25  and affect flow rate characteristics. According to the second embodiment of this invention, however, the slit portion  35  is formed in a manner isolating an organic protective film  30  from an organic protective film formed over the thin-wall portion edge, the organic protective film  30  being positioned in an area where the mold resin  60  is in contact with the thermal airflow sensor. With this structure, the stress does not reach the organic protective film formed over the thin-wall portion edge by way of the organic protective film  30 . This provides an advantage of reducing the stress-induced effects on flow rate characteristics. 
     The third embodiment of the present invention will now be explained with reference to  FIGS. 10 through 12 . The same structures as those of the second embodiment will not be discussed further. 
     As shown in  FIGS. 10 and 11 , during resin molding, a slit inner periphery-side organic protective film  33  is positioned in a manner partially exposed from the mold resin  60 , and a slit outer periphery-side organic protective film  34  is covered with the mold resin  60 . With the slit inner periphery-side organic protective film  33  located in an area partially exposed from the mold resin  60 , even if the mold resin  60  flows out through the gap  61  as shown in  FIG. 12 , the mold resin  60  is stemmed by the protective film  33  and thereby prevented from reaching the thin-wall portion  25 . 
     Whereas the organic protective film  30  protects the AI wiring  40  from corrosive components such as water, there is fear that the organic protective film  30  itself may absorb water and transfer it to the AI wiring  40 . In the structure according to the third embodiment of this invention, the organic protective film  34  covering the AI wiring  40  in the mold resin  60  does not come into direct contact with air. The structure thus prevents corrosion of the AI wiring. Furthermore, the organic protective film  34  is capable of stemming corrosive components such as water coming in through the interface between the semiconductor substrate  20  and the mold resin  60 , thereby reducing the infiltration of corrosive components including water into the AI wiring  40 . Thus in the structure according to the third embodiment of this invention, possible corrosion of the AI wiring  40  is further reduced and reliability is improved accordingly. 
     Moreover, as with the second embodiment, the protective film sandwiched by the mold resin and the semiconductor substrate is independent of the protective film formed over the thin-wall portion. This structure helps lower the stress-induced effects on the thin-wall portion. 
     The fourth embodiment of this invention will now be explained with reference to  FIG. 13 . 
     Whereas each of the slits of the first embodiment are placed distantly over the entire periphery, the effect of preventing the leakage of the mold resin is still obtained even when the slit is formed on one side alone or in one direction only. 
     If it is known beforehand that the insertion die tends to be in uneven contact with the semiconductor substrate  20 , the direction in which a gap is highly likely to occur can be identified. If the slit is formed in that direction, the slit prevents the leaking mold resin  60  from reaching the thin-wall portion  25 , such that the throughput yield can be significantly improved. 
     The same applies to the above-mentioned stress-induced effects. If the stress of the resin is expected to occur in a specific direction through evaluation of actual products and/or through analysis, the slit may be formed in that direction so as to improve the reliability of the thin-wall portion effectively. 
     The fifth embodiment of the present invention will now be explained with reference to  FIG. 14 . 
     Whereas the slit of the second embodiment is shaped as nested circumferences with space interposed, staggered multiple slits formed as shown in  FIG. 14  also provide the effect of preventing the above-mentioned leakage of the mold resin. 
     One object of forming multi-staggered slits is to protect a resistor  37 , formed over the semiconductor substrate, from the impact of dust. Where it is desired, as in the case of the thin-wall portion  25 , to expose a temperature sensor  37  formed over the semiconductor substrate for the sake of better thermal responsiveness, the protective film needs to be formed inside the slit provided in the second embodiment. In this case, multiple slits are formed staggered to prevent leakage of the mold resin  60 . In such a structure, the multi-staggered slits are also effective in preventing the mold resin  60  from leaking. 
     In the first through the fifth embodiments, the organic protective film  30  should preferably be made of polyimide. While the thin-wall portion  25  is subject to high temperatures as a result of the heating resistor  21  being heated so as to measure the flow rate of intake air, polyimide has good resistance to heat and minimizes heat-induced degradation of the material. This makes it possible to improve the strength of a measuring element  1  against the impact of solid particles for an extended period of time. 
     In a structure where the mold resin  60  is applied to the thermal airflow sensor with the organic protective film  30  interposed therebetween, the organic protective film  30  is stressed due to resin contraction after molding. Where the organic protective film  30  is shaped to communicate with the thin-wall portion edge, the stress caused by resin contraction of the mold resin  60  may reach the edge of the thin-wall portion  25  and affect flow rate characteristics. According to the second embodiment of this invention, however, the slit portion  35  is formed in a manner isolating an organic protective film from an organic protective film formed over the thin-wall portion edge, the organic protective film being positioned in an area where the mold resin  60  is in contact with the thermal airflow sensor. With this structure, the stress does not reach the organic protective film formed over the thin-wall portion edge by way of the organic protective film  30 . This provides an advantage of reducing the stress-induced effects on flow rate characteristics. 
     When the organic protective film  30  is made of polyimide, there can be provided a thermal airflow sensor that improves its strength of the insulating film over the thin-wall portion toward dust and yet controls the drop in throughput yield without increase in cost, even with the semiconductor device sealed with the resin in a manner being partially exposed. 
     REFERENCE NUMERALS 
     
         
           1  Intake air 
           2  Semiconductor package 
           3  Housing 
           4  Flange 
           5  Intake pipe 
           6  Auxiliary passage 
           7  Flow rate detection portion 
           8  Connector terminal 
           10  Lead frame (substrate support member) 
           20  Semiconductor substrate 
           21  Heating resistor 
           22  Upstream resistance temperature detector 
           23  Downstream resistance temperature detector 
           25  Thin-wall portion 
           30  Organic protective film 
           31  Organic protective film 
           33  Slit inner periphery-side organic protective film 
           34  Slit outer periphery-side organic protective film 
           35  Slit 
           36  Slit 
           37  Resistor formed over semiconductor substrate 
           38  Temperature sensor formed over semiconductor substrate 
           40  AI wiring 
           50  Bonding wire 
           60  Mold resin 
           61  Boundary between mold resin and thermal flow sensor 
           80  Lower metal mold 
           81  Upper metal mold 
           82  Resin pouring hole 
           83  Insertion die