Abstract:
A method for mounting semiconductor chips includes the steps of: a) providing a semiconductor chip having a surface that has a diaphragm region and a peripheral region, the peripheral region having a mounting region; b) providing a substrate which has a surface having a recess; c) mounting the mounting region of the semiconductor chip using a flip-chip technique onto the surface of the substrate in such a way that an edge of the recess lies between the mounting region and the diaphragm region; and d) underfilling the mounting region using an underfilling component, the edge of the recess being used a demarcation region for the underfilling component, so that no underfilling component is able to get into the diaphragm region. Also provided is a corresponding semiconductor chip system.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a method for mounting semiconductor chips and a corresponding semiconductor chip system.  
       BACKGROUND INFORMATION  
       [0002]      FIG. 7  shows a conventional method of mounting semiconductor chips and a corresponding semiconductor chip system in cross sectional view. In  FIG. 7 , reference numeral  100  denotes a TO8 base produced, for example, from Kovar. Reference numeral  5  is a micromechanical silicon pressure-sensor chip having piezoresistive transformer elements  51  that are accommodated on a diaphragm  55 . To produce diaphragm  55 , a cavity  58  is introduced onto the back of respective silicon pressure sensor chip  5 , for instance, by anisotropic etching, e.g., using KOH or TMAH. Alternatively, diaphragm  55  may also be produced by trench-etching.  
         [0003]     Sensor chip  5  may be made up of a pure resistance bridge having piezoresistive resistors, or may be combined with an evaluation circuit which is integrated, together with the piezoresistors, in a semiconductor process. A glass base  140  made of sodium-containing glass, which is anodically bonded to the back of chip  5 , is used to reduce mechanical stress caused by solder or adhesive  70  by which glass base  140  is mounted on TO8 base  100 . Reference numeral  53  in  FIG. 7  denotes a bonding pad of an integrated circuit  52  (not further shown), the bonding pad being connected via a bonding wire  60  to an electrical connecting device  130 , which in turn is insulated from TO8 base  100  by an insulating layer  131 . Glass base  140  has a through hole  141  which connects cavity  58 , via a through hole  101  of TO8 base  100  and a connecting device  120  affixed thereon, to externally prevailing pressure P. The construction shown in  FIG. 7  is usually also hermetically welded with a metal cap (not shown).  
         [0004]     However, such a construction has the disadvantage that it is cumbersome, and that problems frequently arise with the hermetic enclosing of sensor chip  5 , for instance, because of leaky welding seams or the like. Since the TO8 housing and the silicon have different thermal coefficients of expansion, mechanical stresses are created in response to temperature changes, which are measured as interference signals by piezoresistors.  
         [0005]      FIG. 8  shows another conventional method of mounting semiconductor chips and a corresponding semiconductor chip system in cross-sectional view. This second example provides attaching a sensor chip  5 , via a glass base  140 ′ that has no through hole, to a substrate  1  made of ceramic, and to passivate it using a gel  2  to protect it from environmental influences. Additionally provided above the chip system on substrate  1  is a protective cap  13  that has a through hole  15  for pressure P to be applied. Glass base  140 ′, in this example, also has no through hole, since pressure P is applied from the other side.  
         [0006]     When such a gel  2  is used, the maximum pressure is disadvantageously determined by gel  2 , since gas diffuses into gel  2 , and, if there is a sudden reduction in pressure, gas bubbles are created in gel  2  which destroy gel  2 .  
       SUMMARY OF THE INVENTION  
       [0007]     The method according to the present invention for mounting semiconductor chips and the corresponding semiconductor chip system have the advantage over the known art in that a construction that is simple, cost-effective and insensitive to stress is made possible.  
         [0008]     The present invention utilizes an overhanging type of construction of a sensor chip on a substrate having a recess, with the aid of a flip-chip mounting technique, a mechanical decoupling of the sensor chip being provided by the lateral overhanging.  
         [0009]     Available production processes may be maintained, for the most part, such as the semiconductor process for sensor components and/or evaluation circuit components and for sensor housing parts.  
         [0010]     Electrical die testing in the wafer composite is possible, as is the end-of-production-line adjustment after assembly on the carrier. The method according to the present invention also makes possible a space-saving construction of sensor chip and evaluation circuit.  
         [0011]     According to one example embodiment of the present invention, pluralities of bondpads are provided in the mounting region, which are mounted on the surface of the substrate via a soldering or adhesive connection.  
         [0012]     According to another example embodiment of the present invention, the recess extends to below the diaphragm region. This has the advantage that no foreign bodies are able to become wedged in below the diaphragm region.  
         [0013]     According to another example embodiment of the present invention, the sensor chip is bonded on its rear surface to a glass base. This increases the resistance to bending. Besides, one may enclose a vacuum between the glass base and the sensor chip.  
         [0014]     According to one further example embodiment of the present invention, in the peripheral region, one or more support bases are provided, which are provided lying on the surface of the housing. This support base prevents tilting in response to the flip-chip mounting.  
         [0015]     According to one further example embodiment of the present invention, the substrate is a part of a prefabricated housing. According to yet another example embodiment of the present invention, the housing is a pre-mold housing made of plastic, into which a lead frame is molded. Such housings are particularly cost-effective.  
         [0016]     According to one additional further example embodiment of the present invention, the housing has an annular sidewall region that surrounds the sensor chip, and is closed above the sensor chips by a cover having a through hole.  
         [0017]     According to a further example embodiment of the present invention, an additional semiconductor chip is mounted in the housing, completely molded in. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]      FIG. 1 a  illustrates a first example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system in a side-plane cross sectional view.  
         [0019]      FIG. 1   b  illustrates the first example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system in a top-plane cross sectional view.  
         [0020]      FIG. 2  is a cross-sectional view illustrating a second example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0021]      FIG. 3  is a cross-sectional view illustrating a third example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0022]      FIG. 4  is a cross-sectional view illustrating a fourth example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0023]      FIG. 5  is a cross-sectional view illustrating a fifth example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor-chip system.  
         [0024]      FIG. 6  is a cross-sectional view illustrating a sixth example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0025]      FIG. 7  is a cross-sectional view illustrating a first example of a conventional method of mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0026]      FIG. 8  is a cross-sectional view illustrating a second example of a conventional method of mounting semiconductor chips and a corresponding semiconductor chip system. 
     
    
     DETAILED DESCRIPTION  
       [0027]     In the figures discussed below, identical reference numerals denote identical or functionally identical components.  
         [0028]      FIGS. 1   a  and  1   b  illustrate a first example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system ( FIG. 1   a  shows a side-plane cross-sectional view, and  FIG. 1   b  shows a top-plane cross sectional view).  
         [0029]     In the first example embodiment shown in  FIGS. 1   a  and  1   b , sensor chip  5 ′ is a surface micromechanical sensor chip which is produced, for example, according to the method described in German patent document DE 100 32 579, and which sensor chip has an integrated cavity  58 ′ above a diaphragm region  55 ′.  
         [0030]     Substrate  1  has a recess  11 , next to which sensor chip  5 ′ is mounted in flip-chip technique in an overhanging fashion. For the mounting, bondpads  53  of sensor chip  5 ′ are soldered in mounting region MB onto bondpads of substrate  1 , using a solder or adhesive connection, such as solder balls  26 .  
         [0031]     Mounting region MB also has an underfilling  28  made of an insulating plastic material, and edge K of recess  11 , which lies between mounting region MB and diaphragm region  55 ′, is used as demarcation edge for underfilling  28  during the mounting process. Demarcation edge K ensures that underfilling  28  is not able to get into or under diaphragm region  55 ′. Diaphragm region  55 ′ of sensor chip  5 ′ thereby extends outwards laterally next to strip-shaped mounting region MB, so that the pressure medium is able to reach diaphragm region  55 ′ without hindrance. In diaphragm region  55 ′, sensor chip  5 ′ is passivated on the surface by a layer (not shown), such as a nitride layer, which acts as a secure medium protection. In mounting region MB, sensor chip  5 ′ is protected from corrosion by underfilling  28 .  
         [0032]     An optional support base  36 , provided at the peripheral region of diaphragm region  55 ′ opposite to mounting region MB, is intended to prevent the tilting of sensor chip  5 ′ during the flip-chip mounting. This support base  36  may be provided either on the upper side of chip  5 ′ or on the opposite surface of substrate  1 , and has no solder surface, so that, in this region, sensor chip  5  rests only on the upper side of substrate  1 , but is not firmly connected to it, so that stress influences are avoided in this region.  
         [0033]     In  FIG. 1   b , strip-shaped mounting region MB of sensor chip  5 , having underfilling  28  and the solder balls  26 , is clearly recognizable. Mounting region MB is substantially smaller than the overall surface of sensor chip  5 , which results in a construction having a diving-board configuration. Recess  11  also extends beyond the width extension of sensor chip  5 ′. In this first specific embodiment, recess  11 ′ is developed in substrate  1  as a narrow trench, which does not extend right up to, or below, diaphragm region  55 ′ of sensor chip  5 ′. However, this does not have to be the case, and, in principle, the recess may also extend to below the diaphragm region, as will be discussed later.  
         [0034]     In this example embodiment of the present invention shown in  FIGS. 1   a  and  1   b , the glass base of the conventional configuration shown in  FIG. 7  or  8  may be completely omitted, since the lateral projection of surface micromechanical sensor chip  5 ′ next to strip-shaped mounting region MB already makes possible the diminution of stress that is created by different thermal coefficients of expansion of silicon and glass at the connections using solder balls  26  and underfilling  28 .  
         [0035]     It should be further noted that the construction shown in  FIGS. 1   a  and  1   b  may be packaged in a housing (not shown in  FIGS. 1   a  and  1   b ).  
         [0036]      FIG. 2  shows a cross-sectional view of a second example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0037]     In the second example embodiment, the substrate is part of a pre-mold housing  10  made of plastic, from which there extends laterally a lead-frame  8  molded into it. Substrate housing  10  has a recess  11 , next to which sensor chip  5  is mounted in flip-chip technique in an overhanging fashion. For the mounting, bondpads  53  of sensor chip  5  are soldered onto bondpads of the pre-mold housing  10 , using a solder or adhesive connection, such as solder balls  26 .  
         [0038]     The minimum separation distance of leadframe  8  in the mounting region of sensor chip  5  is usually greater than the minimum separation distance of bondpads  53  on sensor chip  5 . However, since only few bondpads  53  are required on sensor chip  5 , such as four pieces for connection to a Wheatstone&#39;s bridge, they may be placed as far as necessary from one another.  
         [0039]     The mounting region has an underfilling  28  made of an insulating plastic material, and edge K of recess  11 , which lies between the mounting region and diaphragm region  55 , is used as demarcation edge for underfilling  28  during the mounting process. Demarcation edge K has the function already explained in connection with the first example embodiment illustrated in  FIGS. 1   a  and  1   b.    
         [0040]     Here, too, sensor chip  5  is passivated on the surface in diaphragm region  55  by a nitride layer (not shown), which acts as secure medium protection. In the mounting region, sensor chip  5  is protected from corrosion by underfilling  28 .  
         [0041]     Finally, pre-mold housing  10  has an annular sidewall region  10   a , on whose upper side a cover  20  is provided, having a through hole opening  15   a  for the pressure P that is to be applied. Based on the fact that sensor chip  5  is distanced from the upper side of pre-mold housing  10  because of the flip-chip mounting on the side of the peripheral region opposite the mounting region, efficient, non-problematical transmission of applied pressure P to diaphragm region  55  is ensured.  
         [0042]     In the present example embodiment shown in  FIG. 2 , sensor chip  5  is bonded on its rear to a glass base  140 ″, which may be thinner than in the conventional examples according to  FIGS. 7 and 8  that were mentioned at the outset, since the lateral extension of sensor chip  5  next to strip-shaped mounting region already makes possible.the diminution of the stress that is created by the different thermal coefficients of expansion of silicon and glass at the junction with the solder balls  26  and underfilling  28 .  
         [0043]      FIG. 3  shows a cross-sectional view of a third example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0044]     In the third example embodiment shown in  FIG. 3 , sensor chip  5 ′ is also a surface micromechanical sensor chip which was produced, for example, according to the method described in German patent document DE 100 32 579, and which has an integrated cavity  58 ′ above a diaphragm region  55 ′.  
         [0045]     In this third example embodiment, too, the glass base has been completely omitted, which makes possible a particularly space-saving construction, and a correspondingly lower sidewall region  10   a . The mounting using solder balls  26  and underfilling  28  is the same as in the preceding example embodiments shown in  FIGS. 1   a ,  1   b , and  2 .  
         [0046]     In contrast to the preceding example embodiments shown in  FIGS. 1   a ,  1   b , and  2 , cover  20 ′ has a pressure-connecting nipple  21 , in whose through hole opening  15   b  an optional filter  22  may be built, which filter prevents particles or liquid media from reaching the inside of the sensor packaging. Thus, for example, it may be prevented that water gets in, which, if there were frost, could sever explosively and thereby destroy sensor chip  5 ′.  
         [0047]      FIG. 4  shows a cross-sectional view of a fourth example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0048]     In the fourth example embodiment according to  FIG. 4 , a housing is provided, which is a combination of mold and pre-mold housing. In the left part of  FIG. 4 , an evaluation chip  6  is mounted via solder balls in flip-chip technique on lead-frame  8  and is completely encapsulated. In the right part of  FIG. 4 , there is located the pre-mold region, in which sensor chip  5 ′ is subsequently mounted in the manner that has already been explained in detail in connection with  FIG. 3 . Electrical connections between chips  5 ′ and  6  are made via lead-frame  8 , but are not shown in  FIG. 4 .  
         [0049]      FIG. 5  shows a cross-sectional view of a fifth example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0050]     In the fifth example embodiment shown in  FIG. 5 , in contrast to the fourth example embodiment shown in  FIG. 4 , evaluation chip  6  is connected to lead-frame  8  via bonding wires  60 . This arrangement is advantageous especially for the case in which many electrical connections are needed for evaluation chip  6 . For, in this way, the separation distance of bondpads  53  on evaluation chip  6  may be chosen to be small, and that of the corresponding bondpads on lead-frame  8  to be farther apart.  
         [0051]     In this specific example embodiment shown in  FIG. 5 , too, recess  11 ′ is developed in substrate  10 ′ as a narrow trench, which does not extend right up to, or below, diaphragm region  55 ′ of sensor chip  5 . The distance of diaphragm region  55 ′ from the surface of pre-mold housing  10 ′ may consequently be held low, and therefore one should take care, in such an embodiment, that no particles are able to get into the space between diaphragm region  55 ′ and pre-mold housing  10 ′, which could get wedged in there, and could thereby influence the characteristics curve of the sensor chip.  
         [0052]      FIG. 6  shows a cross-sectional view of a sixth example embodiment of the method according to the present invention for mounting semiconductor chips and a corresponding semiconductor chip system.  
         [0053]     In the example embodiment shown in  FIG. 6 , the positioning on lead-frame  8  of sensor chip  5 ′ and evaluation chip  6  is shown. By contrast to the preceding example embodiments, in this sixth example embodiment, two through hole openings  15   a  are provided for the pressure connection in cover  20 .  
         [0054]     Although the present invention has been explained above in the light of specific example embodiments, it is not limited to these, but may also be implemented in other ways.  
         [0055]     In the above example, only piezoresistive sensor structures were discussed. However, the present invention is also suitable for capacitive or other sensor structures, in which diaphragms are used.