Camera module and its method of manufacturing

<u>FIELD OF THE INVENTION</u>

0001The present invention relates to camera modules, in particular those manufactured using techniques known in the field of microelectronics.

<u>BACKGROUND OF THE INVENTION</u>

0002There are many applications for cameras were size is at a premium. Common examples of these are mobile phones and other mobile multimedia devices, though other applications also exist. To satisfy these demands, miniature camera modules are manufactured using microelectronics techniques.

0003Such modules often use sensors made using semiconductor technology, for example CMOS or CCD (charge coupled device) technologies. These sensors come in the form of arrays of sensor cells on a piece of silicon, sometimes in association with some image processing circuitry.

0004These sensors are very sensitive to light and to electromagnetic interference. Stray light can saturate the sensor. Electromagnetic fields, when strong enough, can also disturb the functioning of the detectors cells. The result, in both cases, is the degradation, to a greater or lesser degree, of the image being generated by the sensor.

0005It is, therefore, necessary to protect the sensor and from electromagnetic fields and to ensure that the only light incident on the sensor is that which has passed via the optics.

0006Furthermore, in many situations, there is significant pressure to minimise the cost of the camera module.

0007<figref idref="f0001">Figure 1</figref> represents a cross-section of a miniature camera module 1 manufactured using known techniques.

0008A sensor die 2 has a lower surface to which are attached solder balls 3, for connections to a circuit board (not shown). On a top surface of the sensor die, there is arranged a sensor array 4 having attached to it micro-lenses 5.

0009In order to produce colour sensors, a colour filter mask (not shown) is placed in front of the sensor array 4. This mask contains areas of colour filter placed at the same pitch as the individual sensor cells, such that each sensor cell has its own colour filter in front of it. The colour filters for neighboring cells are of a different primary colour, for example red, green and blue. When the image is processed, the information from each of the groups of neighboring cells, typically four cells, is combined into information for a single pixel of the image. Thus the final image has ¼ the resolution of the actual sensor array.

0010In an area outside the sensor array, attached to the upper surface of the sensor die are small spacers 6. The small spacers are also attached to a lower surface of the glass plate, known as a cover class 7, such that the cover glass 7 is held parallel to and at a small distance above the sensor die 2. The purpose of the cover glass 7 is to protect the surface of the sensor array 4 from particles, particularly those generated during the assemble processing.

0011On an upper surface of the cover glass 7, there is attached a lower end of a spacer element 8. The spacer element 8 has a tubular or box cross-section and contacts the cover glass in an area outside sensor array.

0012To an upper end of the space element 8 there is attached an optical element 9, having focusing means, such as a lens assembly 10. The focusing means 10 is aligned with the sensor array.

0013To an upper surface of the optical element 9 is attached an aperture mask 11 forming an opening aligned with the focusing means 10 and the sensor array 4. The purpose of the aperture mask 11 is to prevent phantom images being created by rays of high incident angle, particularly those passing through the extremities of the focusing means 10.

0014The spacer element 8 has the purpose of placing the focusing means 10 on the optical element 9 at the correct operating distance from the sensor array 4. This distance is a function of a number of parameters such as the overall size of the sensor array 4, the density of cells in the sensor array 4 and the optical properties of the focusing means 10.

0015A shielding element 12 surrounds in the assembly of the sensor die 2, the cover glass 7, the spacer element 8, the optical element 9, and the aperture mask 11. The shielding element 12 is attached to the lower surface of the sensor die by a suitable means such as crimping and conductive glue 13. The top surface of the shielding element 12 has an aperture aligned with that of the aperture mask.

0016The purpose of the shielding element 12 is to prevent electromagnetic interference and to stop light entering the side of the camera module. In order to fulfil these functions effectively, it is both opaque and conductive, and is often made of a metal foil. In order to obtain an effective faraday cage, the shielding element 12 is connected to a ground connection via the conductive glue 13.

0017The presence of the shielding element increases the volume occupied by the camera module, particularly in the x-y directions. Furthermore it represents an additional cost in terms of both the material itself and the time-consuming assembly steps required to put in place. There may also be a yield loss associated with this part of the process.

0018The shielding element 12 represents an increase in weight. In order to mitigate this, it is possible to make the foil thin. However this has the disadvantage of making the shielding element fragile. Also, as described previously, the shielding element is attached by crimping and gluing. The joint is, to a certain extent, fragile.

0019This is of concern for the manipulation steps required for assembly of the camera module 1 on to the printed circuit board, and significant care is taken to avoid yield loss at this step.

0020Traditionally, such camera modules have been assembled as individual units. For example, a spacer element 8 is placed on a sensor die 2 + cover glass 7 sub-assembly and an optical element 9 added thereon. Were these steps to be performed collectively, a significant economy in manufacturing cost could be obtained.

0021The <patcit id="pcit0001" dnum="US20070052827A"><text>US patent application published as 2007/0052827</text></patcit> describes a method of coating the outside of a camera module. However there is no teaching of how to integrate this into an industrial assembly flow, or indeed much detail of such a flow. Nor is there any indication of the impact on the manipulation of the coated camera.

0022It is therefore desirable to provide a miniature camera module which is smaller and more robust. It is also desirable that the camera module be of lower cost in terms of materials and manufacturing, and be easier to assemble onto the printed circuit board of the equipment for which it is intended.

<u>SUMMARY OF THE INVENTION</u>

0023Embodiments described herein address this need by providing a camera module comprising: <ul id="ul0001" list-style="dash" compact="compact"><li>an outer surface;</li><li>a plurality of pixel arrays, each having a bottom surface;</li><li>a plurality of optical elements, at least one of each optical elements being arranged in front of a pixel array</li><li>an aperture, and</li><li>a plurality of colour filters, one of each of said colour filters being arranged in front of one of each of said pixel arrays,</li></ul> wherein the peripheral spacer element is arranged around said module and between each pixel array and each of said colour filters is adapted to transmit a single colour and at least two of said sensor dice have colour filters adapted to transmit different colours.

0024According to an embodiment, the outer surface has metal layer extending from the aperture to at least one of the bottom surfaces so as to be able to contact a connection on at least one of the bottom surfaces.

0025According to an embodiment, the camera module further comprises apertures and metal layer extending between the apertures.

0026According to an embodiment, said sensor arrays are on separate sensor dice.

0027According to an embodiment, surfaces located between said sensor dice have a metal coating.

0028According to an embodiment, all of the transparent elements arranged in front of one of said sensor arrays are able to transmit infra-red radiation, such that said sensor array is able to form images using said infra-red radiation.

0029It is also provided a method of manufacturing a camera module, comprising: <ul id="ul0002" list-style="dash" compact="compact"><li>providing an assembly comprising a sensor dice wafer, a spacer wafer and an optical element wafer, said spacer wafer being placed in front of said sensor dice wafer and said optical element wafer being placed in front of said spacer wafer;</li><li>sawing a top cut every N sensor dice, N being an integer greater than 1, using a first saw blade of a first thickness, proceeding in a direction from said optical element wafer toward said sensor dice wafer, stopping before said sensor dice wafer is reached, and</li><li>sawing a first bottom cut every N sensor dice and aligned with said top cut, using a second saw blade of a second thickness, proceeding in a direction from said sensor dice wafer toward said optical element wafer, and</li><li>placing a metal coating on a surface exposed by said top cut bottom cuts.</li></ul>

0030According to an embodiment, the method further comprises sawing a second bottom cut between each sensor die where said first bottom cut has not been made and stopping said second bottom cut when the spacer wafer is reached.

0031According to an embodiment, said sawing the top cut is stopped in the thickness of said spacer wafer.

0032According to an embodiment, the method further comprises placing a metal coating on the surface exposed by said second bottom cut.

0033According to an embodiment, the step of pacing a metal coating on the exposed surface of said top cut also forms an aperture on an upper surface of said optical element wafer.

0034According to an embodiment, N is equal to 2.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

0035The foregoing and other purposes, features, aspects and advantages of the invention will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings. <ul id="ul0003" list-style="none" compact="compact"><li><figref idref="f0001">Fig 1</figref> represents a known miniature camera module;</li><li><figref idref="f0002">Figs 2a</figref> and <figref idref="f0003">2b</figref> represent an assembly flow according to an embodiment;</li><li><figref idref="f0003">Fig 2c</figref> represents a variant of part of the assembly flow of <figref idref="f0002">fig 2a</figref>;</li><li><figref idref="f0004">Fig 3</figref> represents a cross-section of a camera module according to an embodiment;</li><li><figref idref="f0004 f0005 f0006">Fig 4a to 4c</figref> represent plan and cross-section views of a camera module according to another embodiment; and</li><li><figref idref="f0007">Fig 5</figref> represents equipment incorporating a camera module according to an embodiment.</li></ul>

<u>DETAILED DESCRIPTION OF THE INVENTION</u>

0036In the following description and drawings, same references designate same elements.

0037<figref idref="f0002">Fig 2a</figref> represents a first part of an assembly flow in terms of cross-sectional views of the elements to be assembled.

0038At step S1, the sensor dice 2, the cover glasses 7, the spacer elements 8, the optical elements 9 and the aperture masks 11 are all presented in a collective form equivalent to that of a wafer. The horizontal pitches of the spacer elements 8, optical elements 9 and aperture masks 11 are chosen to correspond to that of the sensor dice, so that satisfactory alignment of the same will be obtained at the end. The wafer of spacer elements 8 is constructed so that vertical centre-lines of the individual spacer elements 8 lie substantially above the centres of scribe lanes 20 of the wafer of sensor dice 2.

0039The wafer of sensor dice 2 and of cover glasses 7 has been previously assembled using a structure of small spacers 6.

0040At step S2, the aforesaid elements are attached together using appropriate attachment techniques. A lower side of the wafer of spacer elements 8 is attached to an upper side of the wafer of cover glasses 7. The wafer of optical elements 9 is attached to an upper side of the wafer of the spacer elements 8.

0041The resulting wafer stack 21 is then mounted onto a support structure 22.

0042The wafer stack 21 must be cut in order to produce the individual units.

0043This part of the process presents two difficulties.

0044Firstly, the materials to be sawn are quite disparate. The wafer of optical elements 9 is often of a mineral glass, the wafer of aperture masks 11 is of a composite material, FR4 for example, the wafer of spacer elements 8 is of an epoxy and the wafer of sensor dice 2 is silicon.

0045Secondly, the vertical distance down from the top of the aperture mask 11 wafer to the point in the wafer of sensor dice 2 is quite large, around 2mm. This is a consequence of the fact that the optical element 9 works best when maintained at a certain distance from the sensor array 4.

0046In particular, this distance is large relative to the width of the cut. This situation becomes more extreme as the size of the sensor die 2 is reduced because the distance between the optical element and the sensor array does reduce in proportion. Indeed, an increase in either tends to lead to an increase in this distance.

0047It has been found that if the ratio of cutting radius (the part of the blade which is unsupported) to blade thickness approaches 20:1, the risk that the blade vibrates increases significantly. Such vibration can lead to damage in the materials such as chipping and cracking.

0048At step S3, partial sawing of the wafer stack 21 is performed. The wafers of aperture masks 11 and optical elements 9 are sawn through, down towards the wafer of cover glasses 7, as shown by the cross-sectional representation of the circular saw blade 23. This is performed in rows aligned above the scribe lanes 20 of the wafer of sensor dice 2. The saw-blade 23 is stopped either in the horizontal parts of the spacer elements 8 or inside the thickness of the wafer of cover glasses 7.

0049The saw blade 23 is chosen to be able to cut the glass of the optical element 9 wafer quickly and cleanly whilst not clogging up too much in the cutting of the aperture mask 11 and spacer element 8 wafers.

0050In order to be sure of avoiding vibration of the saw-blade 23, a typical safe thickness of the saw-blade 23 will be about 250µm. This is much wider than the scribe lanes 20, which are typically of the order of 100µm wide. To adapt the scribe lane 20 width to that of the saw blade 23 would be uneconomic in that it would reduce too much the number of sensor dice 2 per wafer.

0051Thus it is possible to perform the operation in an acceptable time, while obtaining a satisfactory cut.

0052At step S4, the inner surface of the cut is coated with a metal coating 24. It is advisable to place a protective mask over the aperture in the aperture mask 11 beforehand so as to protect the optical element 9 from the metal deposition One of ordinary skill will be able to perform this part of the process.

0053The metallization may be of the order of 5 to 25 µm thick and materials such as layer of copper followed by stainless steel can be used, though other choices are possible as long as the coating 24 is corrosion and scratch resistant. Indeed a non-metallic coating could also be used as long as it was sufficiently conductive and corrosion and scratch resistant.

0054The choice of deposition technique is conditioned by the requirements of step coverage and ability to coat the bottom of the deep groove. Chemical Vapor Phase and Plasma Vapor Deposition (PVD) are possibilities, though one of ordinary skill will be able to make this choice.

0055<figref idref="f0003">Fig 2b</figref> represents a subsequent part of the process.

0056At step S5, the wafer stack 21 has been detached from the support structure 21, turned over and attached to another support structure 26. The wafer of sensor dice 2 and cover glasses 7 are sawn through along the scribe lanes 20 of the wafer of sensor dice 2. The sawing is continued until the full thickness of the wafer of sensor dice 2 and cover glasses 7 has been cut and the cuts so produced communicate with those produced in the previous sawing process.

0057The saw blade 31 (shown in cross-section) is adapted for the cutting of silicon and glass. It will typically be around 50µm thick.

0058At step S6, further metal coating 32 is deposited in a similar manner to the previous deposition. If there are areas of the bottom surface of the sensor dice 2 where coating is not wanted, for example the solder balls 3, a protective mask may be put in place beforehand.

0059It is possible to use the same composition and thickness as that used for the previous coating 24. However this is not a necessity and other choices are possible, as long as they are compatible with the form and surfaces of the groove and sufficient contact with the first coating 24 is made.

0060By placing ground connections on the bottom surface of the sensor die in such positions as to contact the metal coating 32, it is possible to have the metal coatings 24, 32 act as a faraday cage. Furthermore, the metal coatings 24, 32 are opaque so the sensor array 4 is protected from stray light beams. Since there is a join between the coatings 24, 32 at an overhanging edge, it may be useful to make the spacer element also opaque.

0061<figref idref="f0003">Fig 2c</figref> represents an alternative flow for the steps concerning the deposition of the first metal coating 24.

0062The wafer of aperture masks 11 has been omitted.

0063At step S4b, a protective mask layer 40 is deposited. This could be the same as that mentioned previously, but dimensioned to ensure the correct aperture.

0064At step S4c, the metal coating 24 is deposited as described previously and then the protective mask layer 40 is removed.

0065This allows the economizing of the aperture mask 11 and the step of attaching thereof.

0066<figref idref="f0004">Fig 3</figref> represents a cross-section view of a camera module 50, according to an embodiment.

0067The features common to <figref idref="f0001">fig 1</figref> i.e. the sensor die 2, the sensor array 4, the micro-lenses 5, the small spacers 6, the cover glass 7, the spacer element 8, the optical element 9 and the aperture element 11, having been described previously, will not be described further.

0068The shielding element 13 of <figref idref="f0001">fig 1</figref> has been replaced by the metal coating 24, 32 which extents from the aperture mask 11 down to the bottom surface of the sensor die 2. The metal coating 24, 32 is opaque and so protects the sensor array 4 from stray light. The metal coating 24, 32 contacts a ground connection on the bottom surface of the sensor die 2 and so may act as a faraday cage.

0069The sensor die 2 also has conductive columns 52 connecting the top surface to the bottom surface. These may be formed by boring holes through the sensor die with an appropriate technique, and placing conductive material inside the holes. As such the conductive columns are often known as 'thru-silicon vias'. The conductive columns 52 allow connections to the sensor array 4 which are cost effective and do not increase the size of the module.

0070The absence of the shielding element 13 of <figref idref="f0001">fig 1</figref> means that the overall size of the camera module 50 has been reduced. Furthermore the cost of the shielding element 13 itself and that of the step of attaching it have been saved. Also a weight saving has been achieved.

0071The camera module 50 is more easily manipulated for assembly onto the printed circuit board because the exterior no longer presents the fragility associated with the shielding element 13.

0072Furthermore, the external form of the camera module 50 presents, in certain cases, a useful feature, in that the shoulder 51 can make easier the task of the optical alignment system of the equipment used for assembling the camera module 50.

0073<figref idref="f0004">Figs 4a</figref> represents plan view of a camera module having a matrix 60 of individual sensor arrays, produced using the method of <figref idref="f0002 f0003">fig 2</figref>.

0074The features common to the embodiment of <figref idref="f0004">fig 3</figref> will not be described further.

0075Sensor dice 2a, 2b, 2c, 2d (below, not shown in this figure) are arranged in a matrix formation. Each die has a sensor array 4a, 4b, 4c, 4d, above which is arranged an optical element including its focusing means 10a, 10b, 10c, 10d and a corresponding aperture mask 11a, 11b, 11c, 11d.

0076Around the periphery of the matrix there is arranged at spacer element 8, having a shoulder 51. The spacer element 8 has also internal walls 80 between the individual sensor arrays array 4a, 4b, 4c, 4d. The external surface of the peripheral part of the spacer element 8 is coated as described previously with the metal coating 24, 32.

0077In general, CMOS sensors chips have an infra-red (IR) filter arranged in front of them because the sensor elements are very sensitive to IR to the extent that normal daylight levels of IR can impair the sensors performance. This means that it is difficult to use the same camera for daylight and IR imaging.

0078The sensor arrays of this embodiment each have a single colour filter in front of them and one, 60d, does not have the IR filter. The final image is achieved by juxtaposing and combining the images from the three sensor arrays 4a, 4b, and 4c. The resolution of the final image is the same as that of the individual sensor arrays 4a, 4b and 4c.

0079To achieve the same final image resolution with a single sensor array 4 would mean enlarging the sensor array 4 with the consequence of increasing the distance between the sensor array 4 and the focusing means 10. This would then increase the height of the module which is undesirable. This would also impose other constraints concerning the geometry of the focusing means, increasing the cost thereof. Therefore the configuration of this embodiment greatly increases the image resolution without the penalty of height increase, making it suitable for situations where these are at a premium.

0080Furthermore, because one of the sensor arrays has no IR filter, it is able to capture images using the IR part of the spectrum, making the camera module more versatile than classical camera modules.

0081The absence of the IR filter is not obligatory in as much as the possibility of the IR imaging can be sacrificed in exchange for different performance characteristics.

0082<figref idref="f0005">Fig 4b</figref> represents a cross-section view of the camera module 60 of <figref idref="f0004">Fig 4a</figref>.

0083Between the individual sensor dice 2a, 2b, a groove 70 has been cut in the manner described with reference to <figref idref="f0003">fig 2b</figref>. This groove extends upwards to the lower surface of the internal walls 80 of spacer element 8.

0084The internal walls 80 of the spacer element 8 are solid and, preferably opaque. In the situation where the camera module 60 is a 2 x 2 matrix, the cut of step S3, which passes down through the optical element 8 has been performed every alternate row. For the rows above the internal wall 80 of the spacer element 8, the cut has been stopped at the top of the internal wall 80.

0085The outside of the periphery and the interior surface of the groove 70 have received a metal coating 24, 32 in the manner described before. Also the metal coating 24 extends over the upper surfaces of the aperture mask 11a, 11b and of the internal wall 80 of the spacer element 8.

0086The groove 70 between the sensor dice 2a, 2b is to prevent cross-talk between the sensor dice 2a, 2b. It may arise, depending on the characteristics of the sensor arrays 2a, 2b and on the specification of the overall module, that this is not necessary. In this case, the cutting of the groove and the associated coating step can simply be omitted.

0087The arrangement is similar for the other two sensor dice 2c, 2d and when viewed from 90° to the view shown here.

0088<figref idref="f0006">Fig 4c</figref> represents a cross-section view of a variant the camera module 60 of <figref idref="f0004">Fig 4a</figref>. The difference between this embodiment and that of <figref idref="f0005">Fig 4b</figref> resides in that a cut has been also performed down through the internal wall 80 of the spacer element 8, much in the same way as it would for single-array camera modules.

0089<figref idref="f0007">Fig 5</figref> represents a piece of portable equipment incorporating camera modules according to embodiments. Examples of such pieces of equipment are mobile phones and multimedia players.

0090A printed circuit board 70 has mounted on opposite faces a camera module 50 according to the embodiment of <figref idref="f0004">fig 3</figref> and a camera module according to the embodiment of <figref idref="f0004 f0005">figs 4a-4b</figref>.

0091The printed circuit board 70 and the two camera modules 50, 60 is enclosed in a casing 71 having transparent parts 72 arranged in front of the camera modules 50, 60.

0092The foregoing, with its features, aspects and purposes is given by way of illustration and not limitation. Indeed, it is not intended that the embodiments described be considered the only ones concerned by the present invention.

0093Indeed, the method described could be applied, with minor adaptation, to camera modules containing elements not described herewith, or, indeed, those lacking elements, for example the spacer element, the optical element or the cover glass.

0094A 4x4 matrix of sensor arrays has been described. However other rectangular arrays could be produced equally easily.

0095The equipment of <figref idref="f0007">fig 5</figref> has been described with two camera modules placed facing opposite directions, the two modules being according to different embodiments. However it is possible to have more or fewer camera modules, and have them of the same type. Furthermore, these could be placed on the same side of the printed circuit board.

0096Having thus described at least one illustrative embodiment of the invention, various alterations, modifications and improvements will readily occur to those skilled in the art. Such alterations, modifications and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The invention is limited only as defined in the following claims and the equivalent thereto.