Multispectral sensor, functional sensor unit, and method for producing a multispectral sensor

Abstract

A multispectral sensor (16), comprising a number of detector elements (38) each generating a multispectral sensor signal as part of a respective detector channel (44), wherein said detector elements (38) are arranged on a substrate (30) and covered by a passivation layer (46), and a number of optical filters (42) each associated with a number of said detector elements (38) to form one of said channels (44), should be provided that in favour of superior performance and reliability allows to minimize effects of interference ripples in the sensitivity spectrum. In accordance with the invention, this is achieved in that each of said optical filters (42) is deposited on top of a compensation layer (54) positioned on top of said passivation layer (46), wherein the thickness of said compensation layer (54) is chosen such that the effective thickness of the layer system given by said passivation layer (46) and said compensation layer (54) is at least 20 μm.

Description

[0001] Multispectral Sensor , Functional Sensor Unit , and Method for

[0002] Producing a Multi spectral Sensor

[0003] DESCRIPTION

[0004] Technical background of the invention

[0005] The invention relates to a multispectral sensor . The invention more particularly relates to a multispectral sensor comprising a number of detector elements each generating a multispectral sensor signal as part of a respective detector channel , wherein said detector elements are arranged on a substrate and covered by a passivation layer, and further comprising a number of optical filters each associated with a number of said detector elements to form one of said channels . The invention furthermore relates to a method for producing a multispectral sensor .

[0006] Background

[0007] Optical or multispectral sensors are increasingly being used in such diverse areas of technology as smart phones and mobile devices , smart homes and buildings , industrial automation, medical technology, connected vehicles , etc . At the same time sensor data becomes more complex and is expected to meet the requirements for high accuracy . Further, color and spectral light sensing on a chip-scale have various applications in color identi fication, data authentication, spectroscopy, and other industrial and consumer-level optical detection applications . In a number of important applications , in particular for camera applications , such sensors are used as ambient light sensors for so-called "auto white balance" functionality, in order to provide essential background information for appropriate correction functionalities and thereby in general improve imaging quality . Common multispectral sensors are often based on an array of pixels and on-pixel filters for each pixel . For spectroscopic applications the filters can be chosen to have linear independent filter characteristics . In order to accomplish an accurate spectral measurement of a given source , incoming radiation should be known in order to compensate for di f ferent response of individual pixels . This way an amplitude of a spectral signal of a pixel can be used to calculate a spectral reconstruction of a light source under study .

[0008] For proper performance of sensor devices in such broad bands of applications , high precision of the signal processing and signal detection is a typical requirement . In general , the accuracy and reliability of the output signals provided by multispectral sensors may be limited and lowered by a number of factors . In particular, both static and dynamic sources for potential errors in the signals may be of relevance . As an example of static sources for such errors , geometry factors may become relevant . As another source of potential errors or misreadings of the sensor pixels , so-called " interference ripples" may need to be taken into account . The spectral sensitivity of an optical sensor channel , comprising and combining an interference filter and a detector element associated therewith, strongly depends on the combination of the spectral sensitivity of the respective detector element or pixel and the spectral transmission characteristics of the filter element associated therewith . Consequently, the spectral sensitivity and precision in signal processing may be af fected signi ficantly by the properties of the optical stack between filter and the detector element .

[0009] Integrated interference filters are typically processed on the passivation layer of a Si wafer, typically in a CMOS process . Since the transmission and filter characteristics of the filters are mainly geometry driven, and the passivation layer is deposited in the general deposition process , typically an Si02 layer with thickness 6...9 pm, this layer will act as an active interference element in the optical stack and thereby may provide interference ef fects ( so-called "rip- pies" ) that are overlapping the regular transmission characteristic of the spectral filter . In particular, the amplitude of the ef fect on spectral sensitivity is dependent on wavelength but constant for one system ( 20%- 40% ) . The phase and frequency of the interference ripples are defined by the thickness of the passivation layer . In a typical wafer deposition process , this thickness can vary over the surface of the wafer from 6 pm to 9 pm. Especially for parallel or low FOV irradiance conditions , each optical sensor element then may be af fected by di f ferent interference ripple frequency and phase .

[0010] Regarding the measured spectra of dominant light sources such as spikes of a fluorescence lamp, which might have rather narrow parts , the ef fects of ripples in the transmission properties may be signi ficant and change the sensing signal signi ficantly by up to 5% . The ef fect of the ripples thus need to be reduced in particular for use cases of narrow band spectra .

[0011] Summary

[0012] The obj ect of the invention is therefore to provide an improved multispectral sensor unit of the type identi fied above , that helps overcome the deficiencies identi fied above , in a particularly lean and simple setup in order to reduce manufacturing costs . Further, an improved functional sensor unit and a method for its production should be provided .

[0013] With respect to the multispectral sensor, this obj ect in accordance with the present invention is achieved in that the multispectral sensor comprises a number of detector elements each generating a multispectral sensor signal as part of a respective detector channel , wherein said detector elements are arranged on a substrate and covered by a passivation layer, and further comprises a number of optical filters each associated with a number of said detector elements to form one of said channels , wherein each of said optical filters is deposited on top of a compensation layer positioned on top of said passivation layer, and wherein the thickness of said compensation layer is at least equal to the thickness of the passivation layer . More speci fically, in a preferred aspect of the invention the thickness of said compensation layer is chosen such that the ef fective thickness of the layer system given by said passivation layer and said compensation layer is at least 20 pm.

[0014] Further, preferred embodiments are subj ect of the dependent claims .

[0015] The invention is based on the consideration that the interference ripples are strongly dominated by the geometry of the respective optical elements . In particular, the present invention recogni zes the passivation layer, which is a necessary consequence of the widely in use CMOS process , as the key source of the undesired interference ripples . Thus , even though seemingly unef ficient since requiring additional resources , the present invention considers to minimi ze the effects of the ripples by arti ficially modi fying the "ef fective" thickness of the optical element given by the passivation layer .

[0016] In particular, for typical , usual geometries and the passivation layer having a thickness of about 6 ... 9 pm, the resulting ripples will mani fest in the spectral sensitivity spectrum around a wavelength of 550 nm as oscillations with a wavelength of about 17 nm ( for a thickness of 6 pm) , of about 11 nm ( for a thickness of 9 pm) , and of about 5 nm ( for an ef fective thickness of 20 pm) . Thus , for the typical thickness spectrum the wavelength of the oscillations is comparable to the spectral width of certain peaks of important , dominant light sources . The invention in one aspect suggests to eliminate or at least minimi ze the ef fects of the ripples by condensing their oscillations in the spectrum to wavelengths of hal f or even less of said typical peak widths , such that such a peak in the wavelength spectrum covers at least one or even more wavelengths associated with the ripples , thereby averaging out the maj ority of their ef fects . In particular, the wavelength of the oscillations in the spectral sensitivity spectrum may be considered inversely proportional to the thickness of the passivation layer . Thus , doubling the ef fective thickness of the passivation layer will hal f the wavelength of such oscillations . Accordingly, depending on the desired reduction of this wavelength, the ef fective thickness of the passivation layer in one aspect of the invention may be increased by a factor reciprocal to the desired reduction of said wavelength .

[0017] In particular, in one aspect , the present invention suggests that said passivation layer is made of SiCy .

[0018] In one aspect , the invention considers the concept of modi fying or increasing the ef fective thickness of the layer system given by the combination of passivation layer and compensation layer such that the phase of the ripples in the sensitivity spectrum becomes short enough that it is less than the spectral width of given peaks of a selected light source , such as e . g . a fluorescence lamp . In other words : in this concept , the invention considers the ef fective layer thickness a single and suf ficient parameter to be modi fied in order to minimi ze the undesired ef fect of the ripples . In order to further improve the ef fects of this approach, in a preferred embodiment both the passivation layer and the compensation layer in their layout are designed to minimi ze reflection ef fects at their interface . Within this aspect of the invention, in a preferred embodiment the compensation layer is made of a material of similar or preferably more or less identical refractive index as the passivation layer . Preferably, said compensation layer is adapted to match a passivation layer made of Si02 , and / or is a made of a material with a refractive index of between 1 . 45 and 1 . 5 . In further consequence , in another or additional preferred embodiment , if the passivation layer is made of SiCy , said compensation layer also is made of SiO2.

[0019] In a preferred embodiment , the multispectral sensor is used in a functional sensor unit , particular for use in a smartphone or a wearable device . In other words , with respect to the functional sensor unit , particular for use in a smartphone or a wearable device , the obj ect identi fied above is achieved by providing the multispectral sensor as defined previously .

[0020] In an aspect of the invention, the functional sensor unit is manufactures in an integrated chip design, preferably combining the concept identi fied above with modern wafer level chip scale packaging ("WLCSP" ) technologies in order to obtain a reliable , yet cost-ef fective compact and miniaturi zed sensor design using only few components . In an aspect of the invention, the detector unit is designed in WLCP technology .

[0021] In a preferred embodiment , the channels each have a di f ferent transmission characteristics , preferably are linearly independent .

[0022] Further, in yet another aspect of the invention and in view of the intended application or use of detector unit in an ambient light sensor (ALS ) , preferably about 5 to 12 channels with di f ferent transmission characteristics , in particular peak spectral sensitivity, are provided . In view of this preferred number range for the channels , and taking into account the desired at least approximate reconstruction of the detected spectrum in the visible range , in a preferred embodiment the spectral sensitivity of some or each of the channels is of cosine shape , with the Full Width Hal f Maximum ( FWHM) width being approximately equal to the separation between adj acent peaks . In yet another embodiment , the spectral sensitivity of one or each channel may be of Gaussian shape .

[0023] In yet another aspect the invention suggests a camera system, in particular for use in a smartphone or a wearable device , comprising an ambient light sensor having a functional sensor unit of the type identi fied above . With respect to the method for producing a multispectral sensor unit in a CMOS process , in accordance with the present invention the obj ect identi fied above is achieved in that :

[0024] - a number of detector elements is deposited on a substrate ,

[0025] - a passivation layer is deposited on said substrate on top of said detector elements ,

[0026] - a compensation layer is deposited on top of said passivation layer, and

[0027] - a number of filters is deposited on said compensation layer .

[0028] In detail , in aspects of the present invention the compensation layer, which in comparison to the standard CMOS process may be considered an additional element , may be deposited in one of the following preferred modes :

[0029] The compensation layer may be deposited as an additional part of the actual deposition of the passivation layer, thereby ef fectively increasing the thickness of the passivation layer . In this mode , the deposition of the compensation layer may be considered part of the actual wafer process before grinding or planari zation is ef fected . In this option, additional etching may be required in order to get access to the bond pads contacts .

[0030] In another preferred embodiment , a number of individual compensation layers may be deposited onto the passivation layer by structured deposition . In this aspect , the established li ft-of f process and sputtering deposition are combined, resulting in a process similar to the deposition process for the filters . Since in this approach, separate , unconnected "islands" of compensation layer material are deposited, mechanical stress in the substrate and wafer warp are minimi zed . I f higher thicknesses are desired for conceptional reasons , i . e . in view of the intended use case , multilayer stacks of the compensation layer material may be formed in accordance with another aspect of the invention . In yet another preferred embodiment , the compensation layer may be deposited onto the passivation layer as part of the deposition process of the filters . In particular, and in a preferred embodiment , this may be achieved by the definition of an adequately thick SiCt layer, for formation of the compensation layer, as part of the typically sequential deposition process for the filters , in particular as the first layer in the sequence of layer deposition for the filters . In this embodiment , limitations regarding the maximum possible thickness for the compensation layer may apply .

[0031] In summary, the main advantages of the present invention may be seen in that by proper adj ustment of the optical properties of the stack between the detector elements and the filters associated therewith, the use case ef fects of spectral ripples , in particular for low field of view (" FOV" ) interference filters on Si substrates , may be reduced signi ficantly . Accordingly, the structure of the spectral sensitivity of the sensor devices may be kept rather smooth, similar to the ef fective spectral shape for any device . In consequence , superior increased performance in colour measurement and spectral reconstruction may be achieved with such sensors .

[0032] Brief Description of the Preferred Embodiments

[0033] Preferred embodiments and aspects of the invention are described further in connection with a drawing . In this drawing,

[0034] FIG . 1 shows a smartphone in backside view;

[0035] FIG . 2 shows an ambient light sensor in cross section, in particular for use in the smartphone of FIG . 1 ;

[0036] FIG . 3 shows a sensor pixel in an enlarged excerpt of the sensor of FIG . 2 in cross section;

[0037] FIG . 4 shows graphs of the spectral sensitivity of the sensor pixel of FIG . 3 ; FIG . 5 shows graphs of the spectral sensitivity of alternative embodiments of the sensor pixel of FIG . 3 ; and

[0038] FIG . 6 shows a number of embodiments of the sensor of FIG . 2 in an enlarged excerpt of the sensor of FIG . 2 in cross section each .

[0039] Identical parts are labelled by the same reference numerals .

[0040] Detailed Description of the Preferred Embodiments

[0041] FIG . 1 shows a smartphone 1 in backside view . The smartphone 1 is equipped with a camera system 2 , which, as its maj or components , comprises the actual camera 4 , also referred to as the "camera detection system" 4 , and a multispectral sensor 6 , serving as an ambient light sensor 6 , associated therewith . The camera sensor system 4 , the details of which are of lesser signi ficance for the invention disclosed here , and the ambient light sensor 6 are mounted at the back of a common cover glass 8 , which may be the cover glass 8 of the smartphone 1 as such . The ambient light sensor 6 is used mainly for so-called "auto white balance" functionality in the camera system 2 , in order to provide essential background information for appropriate correction functionalities and thereby in general improve imaging quality of the camera system 2 .

[0042] The ambient light sensor 6 , which is shown in cross section in FIG . 2 , comprises a functional sensor unit 10 in the form and design of an optical multispectral sensor chip . The functional sensor unit 10 of the ambient light sensor 6 comprises a housing 12 with a sensor chamber 14 in which the actual optical multispectral functional sensor 16 is positioned . It is noted that the concept proposed herein can be applied for various types of functional sensor chips and optical devices and that the present invention relates to the design of the sensor 16 alone and therefore , within the scope of the pre- sent invention, may very well be used in other applications as in ambient light sensors 6 .

[0043] In order to allow for proper passage of light or radiation to reach the optical multispectral functional sensor 16 through the housing 12 , the housing 12 is provided with an opening or aperture 18 . The aperture 18 may be provided with a di f fuser element 20 .

[0044] The housing 12 of the functional sensor unit 10 of the ambient light sensor 6 is arranged on a substrate or carrier 30 . A cover section or lid 32 forming the aperture 18 and also part of the housing 12 , is located opposite to the carrier 30 and thereby covers the chamber 14 . The carrier or substrate 30 provides mechanical support and electrical connectivity to electronic components which are integrated into the functional sensor unit 10 . In the embodiment shown, the substrate 30 together with the electronic components integrated therein is designed in the so-called wafer level chip scale packaging ("WLCSP" ) technology . In other embodiments , the carrier 30 may comprise a printed circuit board, PCB (not shown) . However, in other embodiments (not shown) the carrier 30 can also be part of the housing 12 , and electronic components may be embedded into the housing 12 by moulding for example . In yet an alternative embodiment , the circuitry may be designed in TSV technology ("through silicon via" ) . A through-chip via is a vertical electrical connection (via ) that passes completely through a silicon wafer or die . The contacting of the chip in this technology is brought to the backside of the chip and processed as a BGA (ball grid array) . The advantage is that everything may be mounted at wafer level ( Si , filter, glass , optic, aperture , di f fuser . . . ) .

[0045] As part of the functional sensor unit 10 , the optical multispectral functional sensor 16 is arranged inside the chamber 14 and on the carrier 30 . In this particular embodiment , the optical sensor 16 is integrated into a single semiconductor sensor die 34 together with other electronics . The optical sensor 16 comprises an array 36 of individual optical detec- tor elements 38 which will be discussed in further detail below . The detector elements 38 may be implemented as photodiodes , for example .

[0046] As further part of the functional sensor 16 , an array 40 of optical filters 42 is arranged in the chamber 14 above or on top of the detector array 36 of detector elements 38 . The detector elements 38 each, individually or in groups , are associated with an associated optical filter 42 having a di f ferent transmission characteristic . Together, the pixels or detector elements 38 and associated filters 42 form a channel 44 of the functional sensor unit 10 . The optical filters 42 determine the transmission characteristic and thereby the spectral sensitivity of the respective sensor pixel 38 and the respective channel 44 ; they may be interference filters such as an optical cut-of f filter, bandpass , long or short pass filter, dielectric filters , Fabry-Perot filters and / or po 1 yme r filters .

[0047] In order to allow for proper passage of light or radiation, the cover or lid 32 of the housing 12 is provided with the aperture 18 . The aperture 18 is positioned above the functional sensor 16 . In fact , the aperture 18 lies within a field of view ( FOV) of the optical sensor 16 . The field of view of the optical sensor 16 includes all points in space from where , at least theoretically, light radiated from an external radiation or light source may traverse towards the optical sensor 16 , e . g . for a fixed detector position and orientation .

[0048] A control unit and / or a measurement unit (not shown) may be integrated into the semiconductor sensor die 34 alongside with the optical functional sensor 16 . The measurement unit can be considered a control unit for the optical functional sensor unit 10 . For example , it may provide sensor signals which are generated by the optical functional sensor 16 . The control unit and / or the measurement unit may be implemented as control logic, state machines , microprocessor and the like . They may also comprise additional components such as analogue-to-digital converters , time-to-digital converters , ampli fiers which too are located in the semiconductor sensor die 34 . The semiconductor die 34 may have a printed circuit board PCB providing electrical communication to the individual components of the multispectral sensor . In operation, incoming radiation can be detected entering through the aperture 18 by means of the multispectral sensor 16 . Each sensor pixel 38 in reaction generates a multispectral sensor signal , respectively . The measurement unit therefore in total provides a set of multispectral sensor signals .

[0049] In general , the accuracy and reliability of the output signals provided by the multispectral sensor 16 may be limited and lowered by a number of factors . In particular, both static and dynamic sources for potential errors in the signals may be of relevance . As an example of static sources for such errors , geometry factors may become relevant . More precisely, the spectral sensitivity of an interference filter based multispectral sensor as used in the embodiment shown depends strongly on the angular distribution of light hitting the respective filter 42 on the detector array 36 . Static ef fects , such as misalignments of the centre of the sensor die 34 with respect to the aperture 18 in the lid, e . g . , may be compensated by a trans fer matrix used for spectral reconstruction .

[0050] As another source of potential errors or misreadings of the pixels 38 , so-called " interference ripples" may need to be taken into account . As mentioned, for each channel 44 , the combination of the spectral sensitivity of the respective detector element or pixel 38 and the spectral transmission characteristic of the filter 42 associated therewith determines the spectral sensitivity of the channel 44 . Accordingly, the spectral sensitivity of an optical sensor channel 44 , comprising and combining an interference filter 42 and a detector element 38 associated therewith, is af fected signi ficantly by the properties of the optical stack between filter 42 and the ( Si ) detector element 38 , the optical stack being defined by the materials and layers positioned therebetween . In devices produced by a typical CMOS process , as for the sensor 16 , this optical stack mainly is given by the SiCy passivation layer that is deposited on top of the detector elements 38 for passivation and protection, and on top of which the material for the filter 42 is deposited .

[0051] Since the transmission and filter characteristics of the filters 42 are mainly geometry driven, and the passivation layer deposited in the general deposition process , typically an SiC>2 layer with thickness 6...9 pm, this layer will act as an active interference element in the optical stack and thereby may provide interference ef fects ( so-called "ripples" ) that are overlapping the regular transmission characteristic of the spectral filter 42 . In particular, the amplitude of the ef fect on spectral sensitivity is defined by the refractive index change on the borders to the filter 42 and to the Si substrate . This amplitude is dependent on wavelength but constant for one system ( 20%- 40% ) . The phase and frequency of the interference ripples are defined by the thickness of the passivation layer . In a typical wafer deposition process , this thickness can vary over the surface of the wafer from 6 pm to 9 pm. Especially for parallel or low FOV irradiance conditions , each optical sensor element 38 then may be affected by di f ferent interference ripple frequency and phase .

[0052] In an aspect of the present invention, the ef fects of these ripples are minimi zed or even eliminated by controlled modification of the passivation layer and the optical stack between the filter 42 and the associated detectors 38 . In particular, the present invention in one aspect suggests to modify the passivation layer by either increasing its thickness to a value high enough to suppress or reduce the ef fects of the interference ripples , or by providing one or multiple additional optically transparent material layers , preferably having similar refractive index to the wafer passivation layer, on the wafer before processing the interference filter 42 . In particular, in one aspect the target of these modi fications is to reduce the ef fect of interference ripples by trans forming them to increased, preferably not detectable , frequency . FIG . 3 shows an excerpt of the cross sectional view of the sensor 16 , in particular an enlarged cross sectional view of a channel 44 of the sensor 16 . The channel 44 , as mentioned before , as its main components comprises the actual detector element or pixel 38 and the associated filter 42 . In particular, the detector element 38 of the respective channel 44 is embedded in the Si-based substrate 30 . As a consequence of the standard CMOS production process , the detector element 38 is covered by a passivation layer 46 made of SiO2 - In standard systems , the filter 42 would then, in an integrated manner, be deposited and processed directly on top of the passivation layer 46 .

[0053] Due to the standards in CMOS deposition technology, the passivation layer 46 typically has a layer thickness of about 6 pm to 9 pm. For further illustration, FIG . 4 shows two diagrams each representing the spectral sensitivity of the channel 44 of FIG . 3 for the case that the filter 42 is deposited directly on top of the passivation layer 46 . In FIG . 4a, the spectral sensitivity is shown for a thickness of the passivation layer 46 of 6 . 04 pm, whereas in FIG . 4b the spectral sensitivity is shown for a slightly di f ferent thickness of the passivation layer 46 of 6 . 14 pm. The di f ference between the thicknesses shown may easily be the result of variations and tolerances of the production process , and thus in real sensor systems the actual situation at a first position may be represented by the diagram of FIG . 4a and at a second position may be represented by the diagram of FIG . 4b .

[0054] Evidently, in both cases , due to interface reflection ef fects at the upper and lower boundaries of the passivation layer 46 and the interference ef fects caused thereby, the basic spectral sensitivity is overlapped by oscillating changes ("ripples" ) in sensitivity characteri zed by peaks 48 at a wavelength of about 6 pm.

[0055] For comparison, FIG . 4 also shows a graph 50 representing the emission spectrum of a typical light source such as a fluo- rescence lamp . Evidently, the emission spectrum shows narrow peaks 52 . In the situation represented by FIG . 4a, due to the interference ripples , the peak 52 of the emission spectrum coincides with a local minimum of the spectral sensitivity, whereas in FIG . 4b, the spectral sensitivity shows a local maximum at the position of peak 52 . This shows the undesired ef fects of the interference ripples , namely that due to those ripples , and in view of typical fabrication tolerances , sensor elements in di f ferent positions of one and the same chip may have entirely di f ferent spectral sensitivity and therefore might produce completely di f ferent signals in response to one and the same light source . Therefore , the ef fects of the ripples should be reduced .

[0056] In order to achieve this goal , in one aspect and in accordance with a key concept , the present invention suggests to modi fy the optical properties of the stack between the detector element 38 and the filter 42 associated therewith such that the wavelength of the oscillations in spectral sensitivity caused by the ripples is signi ficantly lowered, preferably to a value such that in spectral representation multiple ripples fall into the spectral band of individual peaks of a typical light source . This , in aspects of the present invention, may be achieved by either, i f procedurally possible , directly increasing the thickness of the passivation layer 46 to a value of about 20 pm, or by adding a further layer as compensation layer 54 to the stack between detector element 38 and associated filter 42 .

[0057] The sensor 16 in the embodiment shown in FIG . 3 is designed according to the second aspect and thus is equipped with the additional compensation layer 54 . The compensation layer 54 is positioned on top of the passivation layer 46 and underneath the filter 42 . In order to reduce the ef fect of the interference ripples , the compensation layer 54 in accordance with one aspect of the invention is designed such that the interference ripples are shi fted to an increased, preferably undetectable frequency . In a preferred embodiment , the material of the compensation layer 54 is chosen such that it has similar refractive index to the wafer passivation layer 46, thereby minimizing undesired reflection effects at the interface between passivation layer 46 and compensation layer 54.

[0058] In the preferred embodiment shown in FIG. 3, the compensation layer 54, in order to optimize compatibility with the passivation layer 46, is also made of SiO2and has a thickness of about 14 pm, thereby providing an effective total thickness of the layer system comprising the passivation layer 46 and the compensation layer 54 of 20 pm. As a consequence, the interference ripples increase in frequency, and the narrow band part of the light source spectrum is covered by multiple ripples, resulting in significant smoothing in integral spectral detection.

[0059] For a quantitative assessment, the effects can be approximated as follows:

[0060] If, for the regular interference pattern:

[0061] I) => 2*d*n = (m+1) * X2Coincides with:

[0062] II) => 2*d*n = m * X2(corresponds to wavelength of previous order) for properties of passivation layer: d=6000nm= 6pm, n=1.5 from II) => m = (2*d*n) / X2inserted into I) : X2= 2*d*n / (2*d*n / X2+l) For X2= 550 => X2= 567 AX = X2-X2= 17nm

[0063] Same for d = 9000nm => AX = 11.4nm and for d = 20000nm => AX = 5.1nm.

[0064] This is shown in the graph of FIG. 5a; evidently, multiple peaks 48 created by the ripples are within the spectral width of an individual peak 52 of the light source. In an alterna- tive embodiment , the graph for which is shown in FIG . 5b, a total thickness of the layer system of about 50 pm may be provided . In this embodiment , the ef fects of the ripples are mostly eliminated .

[0065] In accordance with one aspect of the invention, the parameters of the compensation layer 54 are chosen in accordance with the requirements given by the individual use case : a higher thickness of the compensation layer 54 or multiple compensation layers 54 may be provided to fully eliminate the ripples ' ef fect regarding the sensing accuracy . For a general sensor design, the use case FOV as well as the targeting spectra may be taken into account for defining the required thickness of the compensation layer 54 .

[0066] The production process for providing the compensation layer 54 may be done in various variations . For a number of embodiments within the scope of the present disclosure , FIG . 6 shows an enlarged excerpt of the sensor of FIG . 2 in cross section each .

[0067] FIG . 6 in each of the embodiments shown in FIGs . 6a, 6b, 6c shows an excerpt of the cross sectional view of the sensor 16 , in particular an enlarged cross sectional view of two channels 44 of the sensor 16 . Each channel 44 , as mentioned before , as its main components comprises the actual detector element or pixel 38 and the associated filter 42 ; in the embodiment shown, the filters 42 of the two channels 44 di f fer from each other in their transmission characteristics in order to provide separate detector channels 44 . As in the previous embodiment , the detector elements 38 of both channels 44 are embedded in the Si-based substrate 30 . As a consequence of the standard CMOS production process , the detector elements 38 are covered by the passivation layer 46 made of SiO2.

[0068] Further, in the embodiment shown in FIG . 6a, the compensation layer 54 also is deposited by full wafer deposition, as a part of the regular wafer processing before grinding or pla- nari zation, thereby ef fectively increasing the thickness of the passivation layer 46 . In this embodiment a rather high quality interconnection between the compensation layer 54 and the passivation layer 46 may be expected, but on the other hand side additional etching may be required to obtain access to the bond pads of the chip for contacting .

[0069] In the embodiment shown in FIG . 6b, individual compensation layers 54 are deposited onto the passivation layer 46 by structured deposition . This may be achieved by a proper combination of the li ft of f process ( for removing the deposition masks ) and the deposition method, i . e . sputtering . This combined procedure is similar to the deposition process for the filters 42 . The benefit of this procedure is a minimi zation of mechanical stress and wafer warp during deposition . As shown in FIG . 6b, multilayer structures for the compensation layer 54 may be provided for higher thickness of the compensation layer system in total .

[0070] In yet another embodiment as shown in FIG . 6c, the compensation layer 54 may be deposited onto the passivation layer 46 as part of the filter deposition process . In particular, this may be achieved by the definition of an adequately thick SiCt layer, for formation of the compensation layer 54 , as part of the typically sequential deposition process for the filters 42 . In this embodiment , limitations regarding the maximum possible thickness for the compensation layer 54 may apply .

[0071] The embodiments of the multispectral sensor 16 and its incorporation into the functional sensor unit 10 discussed herein have been disclosed for the purpose of familiari zing the reader with novel aspects of the idea . Although preferred embodiments have been shown and described, many changes , modifications , equivalents and substitutions of the disclosed concepts may be made by one having skill in the art without unnecessarily departing from the scope of the claims .

[0072] In particular, the disclosure is not limited to the disclosed embodiments , and gives examples of as many alternatives as possible for the features included in the embodiments discussed . However, it is intended that any modi fications , equivalents and substitutions of the disclosed concepts be included within the scope of the claims which are appended hereto .

[0073] Features recited in separate dependent claims may be advantageously combined . Moreover, reference signs used in the claims are not limited to be construed as limiting the scope of the claims .

[0074] Furthermore , as used herein, the term " comprising" does not exclude other elements . In addition, as used herein, the article " a" is intended to include one or more than one component or element , and is not limited to be construed as meaning only one .

[0075] Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a speci fic order . Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise speci fically stated in the claims or descriptions that the steps are to be limited to a speci fic order, it is in no way intended that any particular order be inferred .

[0076] LIST OF REFERENCE NUMERALS smartphone camera system camera detection system ambient light sensor cover glass functional sensor unit housing chamber multispectral sensor aperture di f fuser element substrate lid sensor die detector array detector element array optical filter channel passivation layer peak graph peak compensation layer

Claims

CLAIMS1. A multispectral sensor (16) , comprising a number of detector elements (38) each generating a multispectral sensor signal as part of a respective detector channel (44) , wherein said detector elements (38) are arranged on a substrate (30) and covered by a passivation layer (46) , further comprising a number of optical filters (42) each associated with a number of said detector elements (38) to form one of said channels (44) , wherein each of said optical filters (42) is deposited on top of a compensation layer (54) positioned on top of said passivation layer (46) , and wherein the thickness of said compensation layer (54) is at least equal to the thickness of the passivation layer (54) .

2. The multispectral sensor (16) of claim 1, wherein the thickness of said compensation layer (54) is chosen such that the effective thickness of the layer system given by said passivation layer (46) and said compensation layer (54) is at least 20 pm.

3. The multispectral sensor (16) of claim 1 or 2, wherein said passivation layer (46) is made of SiCy.

4. The multispectral sensor (16) of claim 3, wherein said compensation layer (54) is made of SiCy.

5. The multispectral sensor (16) of any one of claims 1 to4, wherein said compensation layer (54) is made of a material with a refractive index of between 1.45 and 1.5.

6. A functional sensor unit (10) , particular for use in a smartphone (1) or a wearable device, comprising the multispectral sensor (16) of any one of claims 1 to 5.

7. The functional sensor unit (10) of claim 6, in which the channels (44) each have a different spectral transmission characteristic .

8. The functional sensor unit (10) of claim 7, wherein the different transmission characteristics of the channels (44) are linearly independent.

9. An Ambient Light Sensor (6) , comprising the functional sensor unit (10) of any one of claims 6 to 8.

10. Camera system (2) comprising an ambient light sensor (6) having a functional sensor unit (10) of any one of claims 6 to 8.

11. A method for producing a multispectral sensor unit (16) in a CMOS process, wherein:- a number of detector elements (38) is deposited on a substrate ( 30 ) ,- a passivation layer (46) is deposited on said substrate (30) on top of said detector elements (38) ,- a compensation layer (54) is deposited on top of said passivation layer (46) , and- a number of filters (42) is deposited on said compensation layer ( 54 ) .

12. The method of claim 11, wherein said compensation layer (54) is deposited as an additional part of the deposition of said passivation layer (46) , thereby effectively increasing the thickness of the passivation layer (46) .

13. The method of claim 11, wherein a number of said individual compensation layers (54) are deposited onto the passivation layer (46) by structured deposition.

14. The method of claim 11, wherein said compensation layer(54) is deposited onto said passivation layer (46) as part of the deposition process of said filters (42) .

15. The method of claim 14, wherein said compensation layer (54) is deposited as the first layer as part of the sequential deposition process for the filters (42) .

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