Patent Publication Number: US-2011057284-A1

Title: Cmos image sensor having a curved semiconductor chip

Description:
PRIORITY CLAIM 
     This application claims priority to United Kingdom Application for Patent No. 0915473.3 filed Sep. 7, 2009, the disclosure of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention relates to improvements in or relating to digital image sensors, in particular to Complementary Metal Oxide Semiconductor (CMOS) image sensors. 
     BACKGROUND 
     A digital image sensor is a semi-conductor device which may comprise an array of pixels. Each pixel comprises a photosensitive element such as a photodiode in order to convert incident light into photocurrent. The generated photocurrent is then gathered during an exposure time and converted to a voltage. Finally, the voltage is digitized and read out. The most common type of digital image sensor is the Complementary Metal Oxide Semiconductor (CMOS) image sensor. The CMOS image sensor can be used in many environments such as in a camera module. 
     The CMOS image sensor can be integrated in a single chip with a signal processing circuit. The single chip is manufactured with a planar substrate generally made of silicon. This type of manufacturing process provides an image sensor having a reduced size compared to other methods. The planar structure of the CMOS image sensor is not generally adapted to the characteristics of the other elements of the camera module, such as lenses. As a result, the camera module generally includes an additional optical element in order to improve the optical efficiency of the CMOS image sensor. This results in the addition of an element in the camera module which may impact the weight, size and other aspects of the camera module. The addition of this element, however, still does not guarantee an efficient optical performance for the camera module. 
     There is a need in the art to overcome at least some of the problems discussed above. Such an improved CMOS image sensor could be used in a camera and/or mobile telephone. 
     SUMMARY 
     According to one embodiment, a digital image sensor comprises a planar substrate comprising one or more bonding pads on one side. A silicon chip comprising one or more bonding pads is attached on the planar substrate by the one or more bonding pads. The silicon chip as attached to the planar substrate has a curved shape. 
     According to another embodiment, a method for manufacturing a digital image sensor comprises forming a planar substrate; forming a silicon chip; and attaching the silicon chip to the planar substrate. The silicon chip as attached to the planar substrate has a curved shape. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a diagram of a printed circuit board of a camera module, in accordance with an embodiment, given by way of example, 
         FIG. 2  is a cross-sectional view of the printed circuit board as shown in  FIG. 1 , in accordance with an embodiment, given by way of example, 
         FIG. 3  is a flow chart of the process of soldering a silicon chip to the substrate, in accordance with an embodiment, given by way of example, 
         FIGS. 4 ,  5  and  6  are cross sectional views of the printed circuit board during some of the steps of the process as shown in  FIG. 3 , in accordance with an embodiment, given by way of example, 
         FIG. 7  is a graph describing the generation of a stress force caused by the soldering in accordance with an embodiment, given by way of example. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention concerns a silicon chip bonded on a substrate in a specific manner which causes the silicon chip to take on a curved shape to thereby obtain an improved digital image sensor. 
     Referring to  FIG. 1 , a CMOS image sensor printed circuit board  100  is shown. The printed circuit board  100  comprises a planar substrate  110  and several bonding pads (not shown). The substrate  110  is made of a flame resistant material such as FR-4 which is a type of epoxy resin. The bonding pads are made of a conductive metal such as copper. The bonding pads make electrical connections with other bonding pads belonging to external elements such as other chips. 
     A silicon chip  120  is bonded to the surface of the substrate  100 . The silicon chip  120  comprises electronic components forming an image sensor. These components include, for example, imaging pixels and associated metal oxide layers for the circuitry. The silicon chip  120  also comprises bonding pads in order to make electrical connections with other components such as the substrate  110 . The silicon chip has a thickness of between about 50 and 150 microns. 
     As shown in  FIGS. 1 and 2 , the shape of the silicon chip  120  is curved (with reference to its cross-section taken in a direction perpendicular to the surface of the planar substrate  100 ). This feature provides a number of advantages as will become apparent below. The fact that the silicon chip  120  is curved in cross-section gives rise to an improved image sensor. The curved surface can be formed in many different ways and the nature of the curvature can be adapted to different applications. The curved surface enables more effective focusing of any light incident on the silicon chip. In the prior art, one or more additional optical elements are needed to effectively focus the light. In the present invention, the curved shape of the silicon chip  120  takes the place of these additional optical elements. Therefore, the curved shape of the silicon chip  120  allows for a reduction of the optical elements in order to produce an image. 
     Referring to  FIG. 2 , a number of solder balls  130  of different sizes are located between the substrate  110  and the silicon chip  120 . The size of each solder ball  130  is predetermined. The variation in size of the solder balls being responsible for the curve caused to the silicon chip. The application of solder balls  130  between the substrate  110  and a silicon chip  120  is one method by which the curved surface can be produced. It will be appreciated that other methods may also be appropriate such as deforming the silicon using a series of jigs and a deflection member (not shown).  FIG. 2  shows curvature in a longitudinal direction; however it may be appropriate to have changes in the size of solder balls in other directions than along the longitudinal length of the silicon chip in order to give curvature in different directions (for example, curved in both length and width). 
     As shown in  FIG. 2 , the size of the solder balls is different depending on the location of the solder balls relative to the silicon chip  120 . The size of the solder balls decreases from one side of the silicon chip to the center of the silicon chip  120  and increases from the center of the silicon chip to the other side of the silicon chip  120 . Thus, the solder balls are symmetrically arranged on one side of the silicon chip  120 . 
     The process of placing the silicon chip  120  with the solder balls  130  on the substrate  110  comprises the steps as shown in  FIG. 3 . The method of manufacturing makes use of “Flip Chip” technology which requires that the silicon chip  120  is connected to the substrate  110  with the bonding pads of the silicon chip  120  facing the bonding pads of the substrate  110 . In a step  300 , the silicon chip is soldered to the solder balls using the well-known “Flip-Chip” technology. If possible, the solders balls  130  are attached to the bonding pads of the silicon chip  120  to make the bond process more effective. The silicon chip  120  with the solder balls  130  is then placed on the substrate  110  in such a manner that the solder balls make contact with the bonding pads of the substrate  110  as shown in step  310 . As the solder balls are made of a conductive material, the bonding pads of the silicon chip  120  electrically contact the bonding pads of the substrate  110  via the solder balls. In a further step  320 , the silicon chip  120  and the substrate  110  are heated at a temperature around 230° C. which causes the solder balls to attach the silicon chip  120  to the substrate  110 . Thus, the solder balls  130  of the silicon chip  120  are soldered to the bonding pads of the substrate  110 . 
     During the soldering process between the silicon chip  120  and the substrate  110 , a stress force is generated between the substrate  110  and each solder ball  130 . The generation of such a stress force is a time dependant process based on the time taken to solder as shown on the graph of  FIG. 7 . In the graph, 2 seconds after the start of the soldering process, the generated stress force is about 0.3 mN/mm. Then, the stress force reaches a constant value. This indicates a reinforced link between the solder balls  130  and the silicon chip  120 . 
     During the soldering process, as some of the solder balls have different sizes, the stress forces give rise to the curved shape of the silicon chip  120 . As seen in  FIGS. 1 and 2 , during the soldering process the silicon chip  120  is slightly curved or bent. After the soldering process, the silicon chip  120  keeps this position. 
       FIGS. 4 ,  5  and  6  illustrate the steps of the soldering process as described with reference to  FIG. 3 . As shown in  FIG. 4 , solder balls  130  are attached to the silicon chip  120  corresponding to step  300  of the soldering process. As shown in  FIG. 5 , the silicon chip  120  is placed on the substrate  110  corresponding to step  310  of the soldering process. As shown in  FIG. 6 , the silicon chip  120  is soldered to the substrate  110  corresponding to step  320 . During the soldering process of step  320 , the solder balls  130  turn to liquid. As a result, as shown in  FIG. 6 , the solder balls located in the middle of the silicon chip become more liquefied than the outer balls and come into contact with the surface of the substrate  110  pulling down the center of the silicon chip  120 . This gives rise to the stress forces which occur between the silicon chip  120  and the substrate  110 . 
     In another embodiment, the curved shape of the silicon chip  120  can take place by modifying the size of the bonding pads. In this embodiment, the bonding pads of the silicon chip  120  can be increased in diameter compared to the bonding of the substrate  110 . This will increase the surface wetting area and thus results in the center balls having a lower height after soldering. 
     After the soldering process, a specific under fill material such as an epoxy based material is injected between the solder balls in a step  330 . The under fill material will stabilize the link made by the solder balls  130  between the substrate  110  and the silicon chip  120 . The strength of the silicon chip  120 , the solder balls  130  and the substrate  110  is thus further increased. In addition, the under fill material absorbs a part of the stress generated by the stress forces between the substrate  110  and the solder balls  130 . Therefore, the under fill material provides an even distribution of the stress across the substrate  110  and the silicon chip  120 . 
     It should be noted that reference to light is intended to encompass all frequencies of radiation in which a digital image sensor may operate. It should also be noted that the solder balls may be of any appropriate shape as long as they give a difference in height along the intended curve. 
     The digital image sensor thus produced or formed is suitable for use in any device which makes use of a digital image sensor. For example, the digital image sensor may be used in a camera, in camera modules or in a mobile telephone or in any computer related equipment. 
     It will be appreciated that this invention may be varied in many different ways and still remain within the intended scope of and spirit of the invention.