Patent Publication Number: US-2006003096-A1

Title: Apparatus and method for applying adhesive to a substrate

Description:
PRIORITY CLAIM  
      The present application claims priority under 35 U.S.C § 119 based upon European Patent Application No. 04103089.1 filed on Jun. 30, 2004.  
     FIELD OF THE INVENTION  
      The invention concerns an apparatus and a method for applying adhesive to a substrate.  
     BACKGROUND OF THE INVENTION  
      With the mounting of semiconductor chips, epoxy based adhesives are often used in order to attach the semiconductor chip to a substrate. The adhesive has to be applied to the substrate so that, on subsequent placement of the semiconductor chip, an adhesive layer free of air voids occurs distributed as uniformly as possible over the entire surfathrough ce of the chip. Ideally, the adhesive layer extends laterally beyond the edges of the semiconductor chip and also completely embraces the corners of the semiconductor chip. Furthermore, no adhesive must get onto the surface of the semiconductor chip with the electronic circuits. In order to achieve this, depending on the chip format, type of adhesive and other parameters, different “figured” application patterns are used that range from a simple diagonal cross up to multiple branched line patterns.  
      Basically, two methods are known in order to apply such figured patterns. With the first method, known for example from the European patent application EP 928 637, application is done by means of a dispensing nozzle formed with numerous outlets. The outlets of the nozzle are located at a predetermined height above the substrate. The necessary amount of adhesive is ejected by means of a pressure pulse. In doing so, the shape of the ejected amount of adhesive is dependent on the distance between the nozzle and the substrate. With the second method known for example from U.S. Pat. No. 6,129,040, application is done by means of a single nozzle secured to a writing head. The writing head is guided over the substrate with a programmed movement corresponding to the desired adhesive pattern and, in doing so, “draws” the adhesive pattern.  
      The application of the adhesive to the substrate is a tricky process. An important parameter that has to be observed with the greatest accuracy is the distance of the nozzle from the substrate. Before the production process starts, the height of the substrate places has to be measured. This is done in that the height adjustable nozzle is lowered until it touches the substrate place. This contact is detected and saved as the substrate height. This measuring method has several disadvantages:  
      The nozzle often leaves adhesive on the substrate.  
      The measurement is slow because the contact is detected mechanically.  
      For applications with which several semiconductor chips are mounted on top of each other (so-called “stacked die” applications), the height of the surface of each semiconductor chip has to be determined onto which the next semiconductor chip is to be placed. Here there is a great risk that the semiconductor chip is damaged during measurement.  
     BRIEF DESCRIPTION OF THE INVENTION  
      The object of the invention is to develop a measuring method that no longer has these named disadvantages.  
      An apparatus for applying adhesive to a substrate comprises a writing head with a writing nozzle and a camera. The writing head is movable in two horizontal directions x and y and is moved along a predetermined path in the plane spanned by the coordinates x and y in order to write an adhesive pattern on the substrate. The position of the writing head designated as the z height is controlled very accurately by means of a position measuring and control circuit in order that the tip of the writing nozzle can be guided at a predetermined distance Δz 0  above the substrate. In order that the adhesive pattern is applied at the correct position on the substrate, before applying the adhesive, a camera measures the position of the substrate. In accordance with the invention, the apparatus is equipped with a triangulation measuring system with the aid of which the actual z height of the substrate can be determined in relation to a reference height H R  and with means for determining the z position of the writing head in relation to the reference height H R . The triangulation measuring system comprises a laser that emits a laser beam that includes a predetermined angle with the horizontal and uses the camera that is present in any case for determining the position of the point of impingement of the laser beam on the substrate. During a calibration process, the point of impingement of the laser beam on a reference surface is determined and from it the z height of the reference surface calculated, and the z position of the writing head at which the writing nozzle of the writing head touches the reference surface is determined. In production, the position of the point of impingement of the laser beam on the substrate is determined and from it the z height of the substrate is calculated and then the z position of the writing head adjusted according to the measured z height of the substrate such that the tip of the writing nozzle is moved at a predetermined distance Δz 0  above the substrate.  
      The application of adhesive to the substrate according to the invention consists of a calibration process with which the z position of the writing head in relation to the reference height H R  is found out by determining the z height of the reference surface in relation to the reference height H R  by means of the triangulation measuring system and by determining the z position of the writing head at which the writing nozzle touches the reference surface and of the production process of applying adhesive to the substrate with which first the z height of the substrate in relation to the reference height H R  as a constant value H 1  or the z height H 1 (x, y) as a function of the two horizontal directions x and y by means of the triangulation measuring system is determined and with which then the z position of the writing head during the movement of the writing head along the predetermined path for the application of adhesive is controlled to a z height z=H 1 +Δz 0  or z(x, y)=H 1 (x, y)+Δz 0 , respectively, whereby the parameter Δz 0  has a predetermined value.  
      The calibration of the z position of the writing head in relation to the reference height H R  occurs by means of a calibration device. The calibration device comprises a reference surface the z height of which is determined in a first step by means of the triangulation measuring system. In a second step the z position of the writing head is determined at which the writing nozzle touches the reference surface.  
      In a first embodiment the reference surface of the calibration device is deflectable in z direction. The laser and the camera are positioned in relation to the calibration device such that the laser beam falls on the reference surface and that the point of impingement is located within the field of view of the camera. Then the writing head is lowered in z direction until the writing nozzle touches and deflects the reference surface. As soon as the writing nozzle deflects the reference surface the position of the point of impingement of the laser beam shifts. The begin of the shift of the point of impingement is detected by the camera and from the position data of the position measuring and control circuit the z position of the writing head is determined at that moment in time at which the writing nozzle touched the reference surface.  
      In a second embodiment the calibration device comprises a light source that illuminates the writing nozzle from the side and obliquely so that the shadow of the writing nozzle falls on the reference surface of the calibration device. During lowering of the writing head the shadow of the writing nozzle moves. As soon as the tip of the writing nozzle touches the reference surface an end of the shadow and the tip of the writing nozzle coincide. The movement of the shadow is monitored with the camera and from it as well as from the position data of the position measuring and control circuit the z position of the writing head is determined at which the writing nozzle touches or would touch the reference surface. If the angle is known under which the light emitted from the light source falls on the reference surface then it is possible to determine the z position of the writing head at which the writing nozzle would touch the reference surface without that the writing head is lowered until the writing nozzle effectively touches the reference surface. Preferably the writing nozzle is provided with an appendage and then the movement of the shadow of the appendage analyzed.  
      During production the application of the adhesive is carried out in that the position of the point of impingement of the laser beam on the substrate is determined and the z height H 1  of the substrate is calculated. The z height of the substrate can be measured for example at a single location. During application of the adhesive the writing head then takes the z position z=H 1 +Δz 0  wherein the parameter Δz 0  has a predetermined value. The parameter Δz 0  amounts typically to 10 micrometers. Alternatively the z height of the chip mounting area of the substrate can be measured at least at three locations and therefrom the z height H 1 (x, y) calculated as a function of the two horizontal coordinate axes x and y and the writing head guided at the z position z(x, y)=H 1 (x, y)+Δz 0  when applying the adhesive.  
      The writing head performs movements in x- and in y- direction. Because of unavoidable mechanical tolerances the distance between the writing head and the process plate on which the substrate is positioned changes therefore. Therefore it is preferable to determine a correction function by means of a further calibration process that describes the variation of the z position of the writing head in function of the coordinates u 1  to u n , wherein the coordinates u 1  to u n  correspond to the entirety of the degrees of freedom of the writing head and therefore characterize the position of the writing head. The writing head then occupies the z position z′(x, y)=z(x, y)+Δz(u 1 , u 2 , . . . u n ) when applying the adhesive. Naturally one of the coordinates u 1  to u n  equals the coordinate x and another of the coordinates u 1  to u n  equals the coordinate y. Therefore the coordinates u 1  to u n  can alternatively be represented by (x, y, {u}) wherein {u} designates the additional coordinates that characterize the position of the writing head. If the number of degrees of freedom of the writing head in horizontal directions is 2 then {u}={0} (mathematically called an empty set), because the coordinates x and y suffice to describe the position of the writing head. The writing head then occupies the z position z′(x, y)=z(x, y)+Δz(x, y, {u}) when applying the adhesive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING FIGURES  
      The accompanying drawings, which are incorporated into and constitute a part of this specification, illustrate one or more embodiments of the present invention and, together with the detailed description, serve to explain the principles and implementations of the invention. The figures are not to scale. In the drawings:  
       FIG. 1  shows an apparatus for applying adhesive to a substrate,  
       FIG. 2  shows a presentation of geometrical relationships,  
       FIG. 3  shows a shuttle onto which a camera and a laser are secured,  
       FIG. 4  shows a drive system for a writing head,  
       FIG. 5  shows a calibration device, and  
       FIG. 6  shows another calibration device. 
    
    
     DETAILED DESCRIPTION OF ;HE INVENTION  
       FIG. 1  shows a side view of an apparatus for applying adhesive to a substrate  2  that is used as a dispensing station on a semiconductor mounting apparatus known as a Die Bonder. The axes of a Cartesian system of coordinates are designated with x, y and z whereby the two axes x and y run in a horizontal plane. In addition, the x-axis designates the transport direction of a transport device  1  for transporting the substrates  2 . Each substrate  2  has a predetermined number of chip mounting sites  3 ,  3 ′ that are arranged at regular intervals. Optionally, the transport device  1  enables a shifting of the substrate  2  along the y-axis. The transport device  1  first transports the substrate  2  in the transport direction x to the dispensing station where adhesive is applied to the presented chip mounting site  3 ′ and then to a bonding station where a semiconductor chip is placed onto the chip mounting site. Such a transport device  1  is known for example from U.S. Pat. No. 5,163,222. The dispensing station comprises a writing head  4  with a writing nozzle  5 . A drive system enables movement of the writing head  4  in the three directions x, y and z. A suitable drive system that also has a position measuring and control circuit  6  for the precise control of the z height of the writing head  4  is known for example from U.S. Pat. No. 6,129,040. Preferably, the writing nozzle  5  is mounted detachably on the writing head  4 . The writing nozzle  5  has a longitudinal shape: it consists of a longitudinal body  7  with a drill hole  9  running along the longitudinal axis  8  of the body  7  that at the tip opens out in a single outlet  10 . The semiconductor mounting apparatus also comprises a camera  11  for measuring the position and orientation of the chip mounting site  3 ′ presented for the application of adhesive. The optical axis  12  of the camera  11  runs parallel to the z-axis. The optical axis  12  of the camera  11  therefore runs perpendicularly to the chip mounting site  3 ′. With correct focussing of the camera  11 , the entire chip mounting site  3 ′ lies in the plane of sharpness of the camera  11 . The writing nozzle  5  is arranged on the writing head  4  so that its longitudinal axis  8  describes a predetermined angle φ with the z-axis or the optical axis  12  of the camera  11 . The angle φ lies typically in the range of 30° to 60°. In this way, only the tip of the writing nozzle  5  is located in the area of the optical axis  12  of the camera  11 . Part of the writing nozzle  5  is still located in the field of view of the camera  11 . The length of the writing nozzle  5  is dimensioned so that the writing head  4  is located outside the field of view of the camera  11  or at most covers an edge area of the field of view of the camera  11  or of the image delivered by the camera  11 . The writing nozzle  5  is connected to an adhesive reservoir and a pump  13  controls the discharge of adhesive from the writing nozzle  5 . The pump  13  can be integrated into the writing head  5  as in the example shown or arranged stationary on the semiconductor mounting apparatus.  
      In accordance with the invention, the dispensing station is equipped with a triangulation measuring system with a laser  14  and with a calibration device  15  for the determination of the z position of the writing head  4  in relation to a reference height H R . As can be seen in  FIG. 2 , the laser  14  emits a laser beam  16  directed towards the substrate  2  that impinges obliquely on the substrate  2  at a predetermined angle ψ in relation to the horizontal. The angle ψ lies preferably in the range of 30° to 60°. Typically, it amounts to around 45°. When the height H 1  of the surface of the substrate  2  changes, then the position of the point of impingement P of the laser beam  16  on the substrate  2  also changes. The position of the point of impingement P is determined with the camera  11  and the height H 1  of the surface of the substrate  2  is calculated from the position change Δw. The height H 1  of the surface of the substrate  2  results in: 
 
 H   1   =H   R   +Δw *tan(ψ)  (1)
 
 Thereby designate: 
 
 w: a coordinate axis that is defined as the direction of the laser beam  16  in the xy plane, ie, the projection of the laser beam  16  on the xy plane, 
 
 w R : the position of the point of impingement P of the laser beam  16  when the point of impingement P is located at the reference height H R , 
 
 w 1 : the position of the point of impingement P of the laser beam  16  when the surface of the substrate  2  is located at the height H 1  to be measured, and 
 
Δ w=w   1   −w   R .
 
      From the image delivered by the camera  11  one gets the positions p R  and p 1  in pixel coordinates. By means of a calibration measurement therefore, a conversion factor k has to be determined so that the difference Δp=p 1 −p R  calculated in pixel coordinates of the camera  11  can be converted into the absolute travel difference Δw: 
 
Δ w=k*Δp.   (2)
 
 and one gets 
 
 H   1   =H   R   +k*Δp *tan(ψ)  (3)
 
      When the camera  11  as well as the laser  14  are arranged stationary, then the position of the point of impingement P is only dependent on the height H 1  to be measured and the height H 1  can be calculated with the pixel coordinates delivered by the camera  11  according to the equation (3). In many cases however it is inevitable that the laser  14  and/or the camera  11  are moveable in x and/or y direction. In these cases, it is advantageous to use a global system of coordinates (u, v) and to always convert the regulated positions of the laser  14  and the camera  11  as well as the measured positions w 1  and w R  of the point of impingement P of the laser beam  16  into global coordinates (u 1 , v 1 ) or (u R , v R ), respectively, and to calculate the height H 1  from these.  
      In the following, preferred possibilities are explained as to how the laser  14  is to be integrated into the dispensing station.  
      For a dispensing station with which the camera  11  is moveable in y direction, the laser  14  is preferably arranged so that it is moveable in y direction together with the camera  11 .  FIG. 3  shows a plan view of such an example. The camera  11  and the laser  14  are arranged on a shuttle  17  that is moveable in the y direction. Preferably, the position of the laser  14  on the shuttle  17  is adjustable within a predetermined range B so that, if necessary, the direction w can be altered. In addition, with this example a first local system of coordinates (x s , y s ) of the writing head  4 , a second local system of coordinates (x K , y K ) of the camera  11  and the global system of coordinates (u, v) are presented. A position measuring and control system of the writing head  4  controls the position of the writing head  4  in relation to the local coordinates (x S , y S ), a position measuring and control system of the camera  11  controls the position of the optical axis  12  of the camera  11  in relation to the local coordinates (x K , y K ). The global coordinates of the writing head  4  are given by 
 
( u   S   , v   S )=( u   S0   , v   S0 )+( x   S   , y   S ),
 
 the global coordinates of the optical axis  12  of the camera  11  are given by 
 
( u   K   , v   K )=( u   K0   , v   K0 )+( x   K   , y   K ).
 
       FIG. 4  shows a drive system for the movement of the writing head  4  along the three coordinate axes x, y and z. The drive system comprises a first, rigidly arranged guide  18  on which a first shuttle  19  moveable in x direction bears, a second guide  20  arranged on the first shuttle  19  on which a second shuttle  21  moveable in y direction bears, and a third guide  22  arranged on the second shuttle  21  on which the writing head  4  bears, as well as three motors for driving the first shuttle  19  in x direction, driving the second shuttle  21  in y direction and driving the writing head  4  in z direction.  
      If the laser  14  is not arranged rigidly nor arranged moveably together with the camera  11 , then a first possibility exists in securing the laser  14  on the writing head  4  so that the laser  14  follows all movements of the writing head  4  in x, y and z direction. A second possibility exists in securing the laser  14  to the second shuttle  21  so that the laser  14  only follows the movements of the writing head  4  in x and y direction but not in z direction. A further possibility exists in combining the solutions shown in  FIGS. 3 and 4 , i.e. not to arrange the guide  18  shown in  FIG. 3  stationary but to secure it to the shuttle  17  shown in  FIG. 3  and to mount the laser  14  on the slide  21 . With this solution the writing head  4  is movable in the global direction v together with the camera  11  as well as relatively to the camera  11 .  
      With all these three design possibilities, the laser  14  is preferably secured so that it can be rotated on the axis of the laser beam  16  so that the polarisation direction of the laser light can be changed in relation to the substrate  2  and that the direction w is adjustable so that any diffraction effects of the laser light on the substrate  2  can be minimised. Especially with semiconductor chips that have storage elements with numerous structures running parallel to the edges of the semiconductor chip, it can be of advantage when the direction w runs along a diagonal of the semiconductor chip.  
      The z position of the writing head  4  (and therefore also the z height of the writing nozzle  5 ) is controlled by a known position measuring and control circuit that enables control of the z position of the writing head  4  with an accuracy lying in the micrometer range that suffices for the application of adhesive to the substrate  2 . In order that the z height of the tip of the writing nozzle  5  secured to the writing head  4  can be adjusted relative to the surface of the substrate  2 , the relationship still has to be determined between the z position of the writing head  4  and the reference height H R , i.e. that z position Z R  of the writing head  4  has to be determined at which the tip of the writing nozzle  5  is located on the reference height H R .  
      This determination can be carried out in different ways. In a first embodiment the laser  14 , the camera  11  and a calibration device  15  with a reference surface  30  that deflects when the writing nozzle  5  touches it are used. The surface of a deflecting bar  23  for example can serve as the deflectable reference surface  30  as is described in more detail based on  FIG. 5 . The calibration device  15  contains a base  24  with a stop  25 , the deflecting bar  23  and a spring  26 . One end of the deflecting bar  23  bears on the base  24 . The spring  26  presses the other end of the deflecting bar  23  against the stop  25  of the base  24 . The z height H F  of the reference surface  30  is defined by this position of the deflecting bar  23 . The position of the point of impingement P of the laser beam  16  on the deflecting bar  23  is measured by means of the camera  11  and from it the z height HF calculated analogously to equation (3): 
 
 H   F   =H   R   +k*Δp *tan(ψ)  (4)
 
 Here Δp denotes the distance in pixel coordinates of the camera  11  between the position of the point of impingement P of the laser beam  16  when the point of impingement P is on the reference height H R  and the position of the point of impingement P of the laser beam  16  on the reference surface  30  along the projection of the laser beam  16  onto the xy plane. 
 
      Afterwards the writing head  4  is lowered in z direction. As soon as the tip of the writing nozzle  5  touches the deflecting bar  23  and deflects it against the force of the spring  26 , the position of the point of impingement P on the deflecting bar  23  shifts. The z height z S  of the writing head  4  is now determined at that time at which the camera  11  detected the start of the shifting of the position of the point of impingement P. The z height z S  is equal to the height H F : z S =H F .  
       FIG. 6  illustrates a second embodiment with which the calibration device  15  shown schematically and from the side comprises a light source  27  that illuminates the writing nozzle  5  with a light beam  28  from the side and obliquely under the angle θ so that the shadow  29  of the writing nozzle  5  falls on the reference surface  30  of the calibration device  15 . The height H F  of the reference surface  30  determined as with the previous embodiment from the measured position of the point of impingement of the laser beam on the reference surface  30 . During lowering of the writing head  4  the shadow  29  approaches the writing nozzle  5 . As soon as the writing nozzle  5  touches the reference surface  30  an end of the shadow  29  and the tip of the writing nozzle  5  coincide. The movement of the shadow  29  is monitored by the camera  11  and from it and the position data of the position measuring and control circuit the z position z S  of the writing head  4  is determined at which the writing nozzle  5  touches or would touch the reference surface  30 . If the angle θ is known under which the light beam  28  emitted from the light source  27  falls on the reference surface  30  then from the movement of the shadow  29  and the position data of the position measuring and control circuit the z position z S  of the writing head  4  can be determined touchless namely without that the writing head  4  is lowered until the writing nozzle  5  in fact touches the reference surface  30 .  
      During production, the application of adhesive takes place in accordance with the following steps:  
      Determining the height H 1  of the chip mounting surface of the substrate  2  to which the adhesive is to be applied,  
      Approaching the position z=H 1 +Δz 0  with the writing head  4  whereby Δz 0  corresponds to the distance that the tip of the writing nozzle  5  should take up from the surface of the substrate  2 , and  
      Moving the writing head  4  along the predetermined path for applying the adhesive.  
      When the chip mounting surface to which the adhesive is to be applied exceeds a certain size then it is of advantage to determine the height of the surface at least at three locations and to calculate therefrom the height H 1 (x, y) as a function of the two coordinate axis x and y. During the movement along the path the z position of the writing head  4  is then controlled according to its actual position (x, y) to 
 
 z ( x, y )= H   1 ( x, y )+Δ z   0 .
 
      When the writing head  4  is moved in x direction and in y direction then, generally, the distance of the writing head  4  from the process support changes as a purely mechanical planarity of the guides  18  and  20  to the process area can only be achieved with great effort. For this reason, in a calibration process, the deviation Δz(x, y) of the z height of the writing head  4  is determined as a function of the location (x, y). When the laser  14  is secured to the second shuttle  21  of the drive system for the writing head  4  so that the laser  14  only follows the movements of the writing head  4  in x and y direction but not in z direction, then the function Δz(x, y) can be determined for example with the laser  14 . The deviation Δz(x, y) is then used in production as a correction function in order to eliminate deviations caused by the system in that the writing head is guided on the z position z(x, y)=H 1 +Δz(x, y)+Δz 0  or z(x, y)=H 1 (x, y)+Δz(x, y)+Δz 0 .  
      The apparatus in accordance with the invention for applying adhesive to a substrate is not limited to the field of the assembly of semiconductor chips. With the same apparatus, substrates for other components, for example optical components, passive electrical components such as resistors and capacitors, etc, can also be coated with adhesive.  
      While embodiments and applications of this invention have been shown and described, it would be apparent to those skilled in the art having the benefit of this disclosure that many more modifications than mentioned above are possible without departing from the inventive concepts herein. The invention, therefore, is not to be restricted except in the spirit of the appended claims and their equivalents.