A PHOTOSENSITIVE DEVICE AND A METHOD OF MANUFACTURING SAME

Abstract: A photosensitive device (200) has a photosensitive element (D) with a silicon body (1) having first and second opposite conductivity type regions (1 and 7) forming a photosensitive pn junction (7a) and a light receiving surface (SA) carrying at least one of two electrical contacts (11 and 12) coupled to respective ones of the first and second opposite conductivity type regions for extracting charge carriers from the vicinity of the pn junction (7a). An interference filter structure (100) is deposited on the light receiving surface to allow only certain wavelengths of light to pass through to the light receiving surface. An electrode (11a) extends through the interference filter structure (100) to make electrical contact with the at least one electrode (11).

Description: A PHOTOSENSITIVE DEVICE AND A METHOD OF MANUFACTURING SAME This invention relates to a photosensitive device and a method of manufacturing such a device. In particular, this invention relates to a photosensitive device comprising a silicon semiconductor body having opposite conductivity type regions forming a photosensitive pn junction and coupled to electrical contacts to enable extraction of charge carriers generated by incident light in the region of the photosensitive pn junction. The spectral response of a semiconductor photosensitive device is determined primarily by the type of semiconductor forming the photosensitive device. Accordingly, conventionally different types of semiconductor photosensitive devices have been used for different applications. For example, where outdoor lighting is required to switch on or off in response to ambient outdoor light illumination level conditions so that the lighting is switched on only at night or in very low light levels, cadmium sulphide photosensitive devices have conventionally been used because their inherent spectral response is suited to these particular requirements. However, over the past ten years or so, because of issues such as environmental concerns over the use of heavy metals such as cadmium and concerns over the relatively high manufacturing costs required to manufacture cadmium sulphide photosensitive devices, attempts have been made to produce a silicon photosensitive device suitable for use in controlling outdoor lighting such as street lighting. These approaches have primarily involved placing the silicon photosensitive device within a housing having a coloured filter which blocks out infra red electromagnetic radiation. Such coloured filters have generally been formed by incorporating dyes or pigments into the plastics material of the window for the photoelectrical device. It has, however, been found that these dyes or pigments tend to degrade with time so that the photosensitive device no longer produces the required response. Although it is possible to tune the spectral response of a silicon photosensitive device by adjusting the depth of the photosensitive pn junction, this requires precise control over the manufacturing processing, in particular over the implantation and/or diffusion of the dopants to form the pn junction. This may result in low yields of satisfactory devices and therefore increased manufacturing costs. In one aspect, the present invention provides a method of manufacturing a photosensitive device wherein an interference filter is directly deposited onto the light receiving surface of a silicon pn junction photosensitive device which has already been provided with appropriate electrical contacts. In another aspect, the present invention provides a photosensitive device comprising a silicon photosensitive pn junction having a light receiving surface on which has been deposited a Fabry Perot resonator structure adapted to allow substantially only light having a wavelength in the range between approximately 800 and 900 nanometres to be transmitted to the light receiving surface. In another aspect, the present invention provides a photosensitive device comprising a silicon photosensitive pn junction having a cut-off interference filter formed on its light receiving surface. In another aspect, the present invention provides a photosensitive device comprising a silicon photosensitive pn junction having a cut-on filter structure formed on its light receiving surface. The present invention also provides an infra red blind daylight sensor for controlling an outdoor light such as a street light, which comprises a silicon photosensitive pn junction having a cut-off filter structure formed directly onto its light receiving surface so as to inhibit transmission of infra red radiation to the light receiving surface of the sensor. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figures 1 to 4 show schematic cross-sectional views of part of a semiconductor body illustrating various stages in the manufacture of a first photosensitive device in accordance with the invention; Figure 5 shows an enlarged schematic cross-sectional view of a portion of the part of the semiconductor body shown in Figures 1 to 4 to illustrate the formation of an interference filter of the photosensitive device, the interference structure being shown unhatched in the interests of clarity; Figures 6 and 7 show cross-sectional views similar to Figures 1 to 4 of further steps in the manufacture of the first embodiment of a photosensitive device in accordance with the invention; Figure 8 shows a graph of transmission against wavelengths for a cut-off interference filter of the first embodiment of a photosensitive device in accordance with the present invention; Figure 9 illustrates schematically and in part cross-section a photosensitive device embodying the invention encapsulated in a housing; Figure 10 illustrates very schematically one typical use for a photosensitive device embodying the invention and having a cut filter with the transmission characteristics shown in Figure 8; Figure 11 shows a cross-sectional view similar to Figure 5 for illustrating the formation of a different type of interference filter of a photosensitive device embodying the invention; Figure 12 illustrates the transmission characteristics which might be expected from the interference filter illustrated by Figure 11; Figure 13 illustrates very schematically the transmission characteristics of non-monocrystalline silicon; Figure 14 illustrates the actual transmission characteristics of the interference filter shown in Figure 11; Figure 15 is a view similar to Figure 5 illustrating the formation of a further different type of interference filter on a silicon photosensitive element to form another embodiment of a photosensitive device in accordance ith the present invention; Figure 16 illustrates the transmission characteristics of the interference filter shown in Figure 15; and Figure 17 illustrates very schematically apparatus suitable for forming an interference filter with part of the apparatus housing shown cut-away. It should, of course, be understood that Figures 1 to 7, 9 to 11, 15 and 17 are not to scale and that various dimensions, such as the thickness of layers, may have been relatively increased in the interests of clarity. Like reference numerals are used throughout the drawings to refer to like parts. Referring now to the drawings, Figures 1 to 7 illustrate one embodiment of a method of manufacturing a photosensitive device in accordance with the invention. Figure 1 shows a monocrystalline silicon body or wafer 1 having first and second opposed major surfaces 2 and 3. The monocrystalline silicon body is either intrinsic (that is not-intentionally doped) which means it will be generally n-conductivity type or very lowly doped with n conductivity type impurities such as phosphorous or arsenic. Typically, the silicon body 1 is 380 μm (micrometres) thick and has a resistivity of Ik ohm (kilo ohm) to 2k ohm. Impurities are introduced into the second major surface 3 using conventional techniques such as implantation and/or diffusion to form a relatively highly doped n conductivity type region 4 which is typically 5 μm thick and has a resistivity of 3 to 5 ohms. An oxide mask 5 is then defined on the first major surface 2 using conventional photolithographic and etching techniques to form a window 6. P type conductivity impurities, for example boron ions, are then introduced through the window 6 using conventional implantation ar.d/er diffusion techniques to form a planar relatively highly doped p conductivity type region 7 as shown in Figure 2 forming a photosensitive pn junction 7a with the lowly doped n-conductivity type region 1 at the desired depth. Typically, the p+ region 7 is formed by implanting boron ions at 50 kV with a dose of 5 x IO 15 ions cm "2 so that the photosensitive pn junction 7a is typically 1.3 μm below the surface 2. Of course, the actual depth of the pn junction 7a will be tailored to meet the desired photosensitive properties. The peak spectral response of the above desired structure should be about 920 nm (nanometres). The structure thus forms a vertical p-i-n diode. A passivating layer 8, generally a further silicon dioxide layer deposited using conventional chemical vapour deposition techniques or the like, is then provided over the surface 2 to cover the window 6 as shown in Figure 3. The passivation coating may be, for example, 200 nm (nanometres) thick. A mask 9, for example a resist mask, is then defined over the passivating layer using conventional photolithographic techniques to define a window 9a through which a contact hole 10 is etched in the passivating layer 8. As shown in Figure 4, metallisation, for example an aluminium/silicon 99:1 ratio alloy, is then deposited to, for example, a thickness of 1 μm and patterned using conventional techniques to define an anode electrical contact 11 of the photosensitive device. Metallisation is also deposited onto the other major surface 3 to form a cathode electrical contact 12. Generally, the cathode metallisation comprises a silver back coating although, of course, any other suitable electrically conductive materials could be used. The n and p conductivity regions 4 and 7 may have any suitable conventional doping concentrations. The completed silicon photosensitive p-i-n diode or element D shown in Figure 4 may be, for example, one of the S.M.P. range of discrete silicon p-i-n photodiodes marketed by Semelab Pic of Lutterworth, Leicestershire, England, for example the SMP400G, SMP525G, SMP550G, SMP600G, SMP690G, SMP900G, SMP1000G or SMP2000G which differ mainly in die size with the SMP400G having a 1 x 1 mm die and the SMP2000G having a 12 x 12 mm die. As will be appreciated by those skilled in the art, although Figures 1 to 4 show only the manufacture of a single photosensitive p-i-n diode D, generally a large number of photosensitive diodes will be formed at the same time in the semiconductor body or wafer 1. Normally, once the electrical contacts 11 and 12 have been provided as shown in Figure 4, the semiconductor body or wafer would be sliced or diced using an appropriate cutting technique to separate the completed photosensitive p-i-n diodes into separate discrete devices which would then be encapsulated in appropriate packages using conventional semiconductor packaging technology. However, in a method embodying the invention, the semiconductor body or wafer 1 carrying the functionally complete photosensitive diodes D is then subjected to an appropriate cleaning process to remove contaminants such as particles of resist, dust and the like. For example, this cleaning process may comprise a multi-stage, for example four-stage, trichloroethylene cleaning process wherein the semiconductor body 1 is cleaned in successive baths with the cleaning fluid for each bath being provided by the next succeeding bath, so that the purity of the cleaning purity increases with each successive bath. After the semiconductor body has been dried, it is then transferred to a thin film vacuum deposition apparatus, such as that manufactured by Balzers. An example of such apparatus is illustrated very schematically in Figure 17. As shown, the vacuum deposition apparatus 30 comprises a vacuum chamber 31 in the form of a bell jar 31a which is mounted to and is removable from a base 31b coupled via a vacuum pipe 34 to a diffusion pump 35 and thence via a pipe 34a to a rotary pump (not shown). The base 31b also has a gas inlet 33 for supplying gas to the chamber via an appropriate valve (not shown). A domed substrate holder 34 is mounted on a frame 35 supported on, for example, three pulley wheels 35a one of which is driven by a motor (not shown) provided outside the vacuum chamber 31 to rotate the frame 35. The semiconductor body 1 or substrate is mounted to an aperture in the domed holder as shown schematically in Figure 17. A lamina or shield 36 may be mounted to the base of the chamber 31 on a support 36A so that the lamina 36 is just below the substrate 1. Evaporation sources for the materials to be deposited onto the wafer 1 are provided by a multi-hearth electron beam gun. For simplicity, the electron beam gun and associated sources are indicated simply by the block 37 in Figure 17. The purpose of the lamina 36 is to increase the uniformity of thickness of a thin film layer deposited on the wafer 1 by electron beam evaporation. The actual shape of the lamina may be computer-generated by using appropriate known integration techniques using the predetermined angular spread φ of material evaporated from the hearths of the electron beam gun. Other deposition techniques may be used, for example thermal evaporation or sputtering may be used to deposit the thin flow. As another possibility, a chemical vapour deposition process may be used. Once the wafer 1 has been loaded into the vacuum chamber 31 in conventional manner, the chamber 31 is evacuated using the vacuum pump (not shown) to a pressure of about 5 x 10" s torr and the wafer substrate heated to about 240°C by a heater (not shown) mounted to the neck of the bell jar 31a. Immediately prior to commencement of deposition, the substrate 1 is further cleaned by inert ion bombardment from a dc plasma generated between the conductive parts of the chamber and an electrode 38 provided in the floor of the chamber. Successive thin film dielectric layers are then deposited over the electrical contact 11 and passivating layer 8 by evaporation from the multi-hearth electron beam gun 37. The evaporation rate from the multi-hearth electron beam gun is controlled in conventional manner through the use of quartz crystal controllers QC which use the rate of change in the resonant frequency (about 6MHz) of quartz crystals to measure the evaporation rate, so allowing the rate of evaporation to be controlled by controlling the electron beam gun. This allows the stoichrometry of the thin films, which is dependent on the evaporation rate, to be controlled. The thickness of a deposited layer may be determined from the rate of evaporation. However, in the present case an optical detection arrangement is used. A light source (generally a laser) and optical system 39a is coupled to the base of the chamber 31 to direct light through the chamber onto an optical glass 390b of an optical glass holder 39b mounted to the top of the bell jar above a central aperture 34a of the dome 34. A detection arrangement 39c is similarly coupled to the base 31b. The detection arrangement 39c may comprise a photomultiplier having a removable narrow band pass filter or variable wavelength monochromator positioned ir. front of its input window so that the photomultiplier is responsive to a selected wavelength. In use, as material is deposited onto the substrate, it is also deposited onto the optical glass 390b. The amount or intensity of light at the selected wavelength reflected by the optical glass depends upon the thickness of the deposited layer with the intensity of light detected by the photomultiplier passing through a peak (maximum or minimum) every time the thickness is an integral number of quarter wavelengths of the selected wavelength. Thus, by appropriate selection of the selected wavelength, the layer thickness can be determined by stopping evaporation at an appropriate maximum or minimum in the output of the photomultiplier. The optical glass holder is designed to allow different optical glasses to be selected, so enabling a clear optical glass to be used after each different layer or combination of layers. Similarly the selected wavelength may be changed by adjusting the monochromator or changing the narrow band pass filter. As is conventional, actuation and selection of the appropriate hearth of the electron beam gun may be computer or manually controlled. An example of the formation of a cut-off interference filter structure 100 onto the already formed photosensitive p-i-n diode D will now be described with reference to Figure 5 which shows a cross-sectional view on an enlarged scale of part of a completed photodiode D of the semiconductor body 1. In this example, a thin film layer 12 of titanium dioxide is first deposited to a nominal thickness of 109.1 nanometres (λ 0 /4 where λ 0 is the wavelength in air to which the photomultiplier is arranged to respond and is in this case lμm). In order to achieve a stoichiometric (and therefore minimally absorbing) thin condensed film, oxygen is introduced into the vacuum chamber 31 through the gas supply line 33 at a partial pressure of about 5 x 10 "4 torr during deposition. The hearth of the multi-hearth electron gun 37 is then changed so as to commence deposition of a thin film layer 13 of silicon dioxide. This layer 13 is deposited to a nominal thickness of 172.2 nanometres (λ 0 /4 where λ 0 =lμm) . Further alternate layers 12 and 13 of titanium dioxide and silicon dioxide of 109.1 and 172.2 nanometres nominal thickness, respectively, are then deposited to build up a stack of alternate titanium dioxide and silicon dioxide layers. This stack has a rejection band centred around lOOOnm. Although not specifically shown in Figure 5, in this example, the stack comprises seven titanium dioxide layers and six silicon dioxide layers. The number of alternate layers 12 and 13 may be adjusted to vary the degree of rejection and the sharpness of the wavelengths at which rejection commences and ends. After deposition of the final titanium dioxide layer 12, a silicon dioxide layer 14 with a nominal thickness of 163.3 nanometres (λ 0 /8 where λ 0 =lμm plus λ 0 /8 where λ 0 =898nm) is then deposited followed by a titanium dioxide layer 15 of 97.9 nanometre (λ 0 /4 where λ 0 =898nm) nominal thickness, a silicon dioxide layer 16 of 147.4 nanometres (λ 0 /8 where λ 0 =898nm plus λ 0 /8 where λ 0 =816nm) nominal thickness and a titanium dioxide layer 17 of 89 nanometres (λ 0 /4 where λ 0 =816nm) nominal thickness. After deposition of the titanium dioxide layer 17, alternate layers 18 and 19 of silicon dioxide and titanium dioxide with nominal thickness of 140.4 nanometres (λ 0 /4 where λ 0 =816nm) and 89 nanometres (λ 0 /4 where λ 0 =816nm), respectively, are deposited to form a second stack of alternate silicon dioxide and titanium dioxide layers. This provides a rejection band centered around 816nm. Again, the number of alternate layers 18 and 19 may be reduced or increased with a larger number of alternate layers 18 and 19 potentially providing a more precise cut-off wavelength. Although not specifically shown in Figure 5, in this example, seven silicon dioxide layers 18 and seven titanium dioxide layers 19 are deposited. A final silicon dioxide layer 20 with a nominal thickness of 70.2 nanometres (λ 0 /8 where λ 0 =816nm) is then deposited. The titanium dioxide and silicon dioxide layers 12 and 13 form a first quarter wavelength stack for light having a wavelength of about 1000 nanometres while the silicon dioxide and titanium dioxide layers 18 and 19 form a second quarter wavelength stack for light having a wavelength of about 816 nanometres. The main role of the layers 14 to 17 is to provide impedance matching between the first and second quarter wavelength stacks. The layers 14 to 17 also enhance the rejection at the region where the rejection bands of the first and second quarter wavelength stacks meet. In accordance with Herpin's theorem any symmetric three layer structure (triplet) can be considered mathematically equivalent to a single layer having, for a particular wavelength, a Herpin index which is inversely related to the impedance of that three layer structure. A portion of the thickness of the silicon dioxide passivating layer 8 and of the silicon dioxide layer 14 can be considered to form layers of thickness λ/8 for the quarter wavelength stack formed by the layers 12 and 13. Similarly, the layer 20 and a part of the thickness of the layer 17 can be considered to form λ/8 thickness layers for the quarter wavelength stack formed by the layers 18 and 19. Thus, the layers 8, 12, 13 and 14 can be considered -co form one series of Herpin layers with a given Herpin index while the layers 17 to 20 can be consid ed to form a second series of Herpin layers with a different Herpin index. The layers 14 to 17 form a coupling structure in the form of a third series of Herpin layers with a Herpin index in between that of the layers 8 and 12 to 14 and the layers 17 to 20 for the pas band of the interference filter structure. These Herpin layer structures are arranged so that Herpin index varies gradually from the passivating layer 8 to air so as to optimise transmission and avoid reflectance of light falling within the pass band of the interference filter structure. Further sets of triplets may be provided between the photodiode passivating layer 8 and the first quarter wavelength stack and on the second quarter wavelength stack to improve further the impedance matching, for example, a further quarter wavelength stack the same as the first quarter wavelength stack may be provided on the second quarter wavelength stack. As shown in Figure 5, the thin film layers deposit uniformally and so follow the surface topography of the underlying photodiode. However, typically, the completed photosensitive diode will have an area of, for example, 100 x 100 micrometres or 10 x 10 micrometres and any steps on the surface will generally be much larger than the wavelength range in which the photosensitive diode is intended to operate. Furthermore, the thin film layers will be substantially flat over the majority of the light sensitive surface SA which lies over the photosensitive pn junction 7a as shown in Figure 5. The surface topography of the semiconductor diode should therefore not cause any problems for the formation of the interference filter structure 100. If, however, smaller dimension photosensitive diodes are to be produced or a flatter surface for formation of the interference filter is desired, • the surface of the completed photosensitive diode D may be planarised using suitable conventional techniques. For example a further silicon dioxide layer may be deposited over the layer 8 and contact 11 followed by a photosensitive resist which provides a planar surface. The resist and further silicon dioxide layer may then be etched using a process which etches the resist and silicon dioxide at the same rate until exposure of the surface of the electrical contact 11 is detected using conventional means. After formation of the interference filter structure 100, a suitable masking layer 21 is deposited over the interference filter structure 100 and a window 21a defined in the masking layer 20 using appropriate conventional techniques as shown in Figure 6. It will be appreciated that, although the interference filter structure 100 is shown in Figure 6 as a single un-hatched layer, this is simply in the interests of clarity and the interference filter structure 100 will, of course, consist of the multi-thin film layer structure described above. The window 21a in the masking layer 21 may be aligned with the electrical contact 11 using conventional mechanical or optical mask alignment techniques because, at the wavelengths which the interference filter structure 100 is designed to transmit, the electrical contact 11 is clearly visible through the interference filter structure. As is well known in the art, the window 21a may be oversized, that is made larger than the surface area of the electrical contact 11 to ensure electrical contact to the contact 11 by subsequent metallisation even if there is a slight misalignment of the mask window 21a. Once the mask window 21a has been formed, a suitable etching process, for example a reactive ion or other anisotropic etching process, is used to etch through the int rf ence filter structure 100 to define a contact hole 100'. The end point of the etching process may be determined by detecting electrically conductive material etched away just as the surface of the electrical contact 11 is exposed. The masking layer 21 is then removed using conventional techniques and, as shown in Figure 7, further electrically conductive material, generally again aluminium, is d< ^osited and patterned using conventional photolithograph and etching techniques to define an anode electrode la so as to complete the photosensitive device 200. The array of photosensitive devices formed on the semiconductor wafer 1 may then be sliced up into individual photosensitive dies ready for encapsulation in appropriate packaging. The fact that the interference filter structure 100 is deposited over an all-ready electrically complete photosensitive diode 200 means that it is not necessary to modify the photosensitive diode manufacturing process which would be costly and time consuming. Rather, a wafer containing photosensitive diodes produced using an existing conventional manufacturing process may be used. This enables the same photosensitive diode manufacturing process to be used to produce photosensitive devices with and without interference filters without significant additional cost to the photodiode manufacturing process. Also, the same photosensitive diode process may, of course, be used for producing photosensitive diodes for carrying different forms of interference filters. In addition, because the photosensitive diodes are electrically complete before the interference filter is deposited, electrical testing of the photosensitive diodes D on a wafer 1 may be carried out before depositing the interference filter structure 100. Figure 8 shows a graph of the transmission T against wavelength λ in nanometres (nm) determined, from computer simulation, to be provided by the interference filter structure 100 described above with reference to Figure 5. As will be appreciated, a transmission T of 1 indicates that all light of that wavelength is transmitted through the interference filter while a transmission T of zero indicates that light at that wavelength is not transmitted but is reflected or absorbed by the interference filter structure 100. As can be seen from Figure 8, the interference filter structure described above with reference to Figure 5 provides a cut-off filter which has a relatively sharp cut-off at about 750 nanometres which is in the red area of the visible spectrum. Accordingly, infra red light is not transmitted by this interference filter. It will, of course, be appreciated that the actual cut-off wavelength may be adjusted or fine-tuned by substantially adjusting or scaling the layer thicknesses. A photosensitive device embodying the invention having the interference filter structure 100 described with reference to Figure 5 is particularly suited for use in automatically switching on or off outdoor lighting such as street lighting because the interference filter structure 100 prevents infra red radiation, to which a silicon photodiode is normally very responsive, from reaching the photosensitive pn junction 7a. Thus, a photosensitive device 200 comprising a silicon photodiode D onto which is deposited an interference filter structure 100 of the type described above with reference to Figure 5 may be used in place of the conventional cadmium sulphide photosensors used in street lighting without the need for coloured filters which may degrade with time. Figure 9 shows very schematically a photosensitive device 200 e bodyinπ the invention mounted within a conventional plastics housing 22 having a clear plastic or glass window 23. As shown in Figure 9, the photosensitive device 200 may be encapsulated together with a separate integrated circuit for example a commercially available ASIC (application specific integrated circuit) 34 with its cathode contact 12 connected via an appropriate conductive track 25 and its anode electrode 11a connected via, for example, a wire bond 26 to appropriate connections on the circuitry 24 which, in conventional manner, controls switching on or off of a light or lamp in response to the output of the control assembly 200' . The encapsulated or packaged control assembly 200' may, as shown schematically in Figure 10, be mounted on top of the support post 40 of a street light 41 so as to automatically control switching on of the street light 41 when it becomes dark and switching off of the street light 41 when it becomes light. Such a control assembly 200' may, of course, also be used for other outdoor lighting such as security lighting for commercial buildings and domestic housing. The photosensitive device 200 may also be used in an automatic camera so as to control whether or not the electronic flash of the camera is actuated in dependence on the detected light levels. One particular type of interference filter structure 100 has been described above. However, other forms of interference filter structure may be provided onto the already formed photosensitive diode by using the method described above with reference to Figure 5 but modifying the materials and thicknesses of the thin film layers used to form the interference filter structure. Figure 11 is a cross-sectional view similar to Figure 5 showing a different interference filter structure 100a formed on top of an already completed silicon photodiode D. In this example, a first layer 50 of non- monocrystalline, generally amorphous or polycrystalline, silicon having a nominal thickness of 56.4 nanometres (λ 0 /4 at λ 0 =830nm) is deposited onto the silicon dioxide passivating layer 8 and contact 11 of the photodiode D followed by a layer 51 of silicon dioxide having a nominal thickness of 285.7 nanometres (λ 0 /2 where λ 0 =830nm), then a further layer 50 of silicon of the same thickness as the preceding silicon layer followed by a second layer of silicon dioxide 51a having a nominal thickness of 142.8 nanometres (λ 0 /4 where λ 0 =830nm) . The structure formed by the three layers 50, 51 and 50 is then repeated one or more times with each three layer structure 50, 51 and 50 being separated from the next three layer structure by a layer 51a. Non- monocrystalline silicon has a refractive index in the range of from about 3.68 to 4 while silicon dioxide has a refractive index of about 1.45. Accordingly, the three layer structures 50, 51 and 50 form Fabry Perot resonators for light having a wavelength of about 830 nanometres. In this example, although not specifically shown in Figure 11, the interference filter structure 100a comprises four Fabry Perot resonators coupled together by the layers 51a. Thus, the interference filter structure 200 is given by: (H2LH)L(H2LH)L(H2LH)L(H2LH) where H represents a layer of the higher refractive index silicon having a nominal thickness equivalent to a quarter wavelength at about 830 nanometres while L represents a layer of the lower refractive index silicon dioxide h v ng a nominal thickness equivalent to a quarter wavelength at about 830 nanometres. Figure 12 shows the transmission characteristics which would be expected of such a coupled Fabry Perot interference filter structure 100a. As can be seen from Figure 12, in addition to the desired pass band A between about 800 and about 900 nanometres, the interference filter structure would be expected to have side bands SB below about 700nm. Figure 13 illustrates diagrammatically the transmittance of lμ of non-monocrystalline silicon deposited by evaporation. Figure 13 neglects interference effects. As can be seen from Figure 13, this non- monocrystalline silicon is highly absorbing up to about 700 nanometres. The use of non-monocrystalline silicon in the band pass interference filter structure 100a therefore prevents transmission through the interference filter structure below about 700 nanometres and so avoids the side bands shown in Figure 12. Accordingly, the use of non-monocrystalline silicon results in the interference filter structure 100a having the transmission characteristic shown in Figure 14. The interference filter structure 100a thus passes or transmits light having a wavelength in the range of from about approximately 800 to about approximately 900 nanometres with only a small, virtually negligible, transmission in the region of 550 to 600 nanometres. Thus, the use of non-monocrystalline silicon in the interference filter structure 100a avoids the need for further filters to remove the side bands below 700 nanometres shown in Figure 12. It will, of course, be appreciated that the actual pass band A' of the interference filter structure 100a may be adjusted by appropriate adjustment of the thickness of the silicon and silicon dioxide layers while still avoiding any undesired side bands below 700 nanometres. After formation of the filter structure 100a, the further steps described above with reference to Figures 6 and 7 are carried out to complete the photosensitive device. Figure 15 is another cross-sectional view similar to Figure 5 showing another form of interference filter 100b which may be formed onto an already produced silicon photodiode D. In this example, a first layer 60 of silicon having a nominal thickness of 16.3 nanometres (λ 0 /8 where λ 0 =500nm) is deposited over the passivating layer 8 and contact 11 of the photodiode D followed by a layer 61 of silicon dioxide having a nominal thickness of 85.5 nanometres (λ 0 /4 where λ 0 =500nm) then a layer 62 of silicon having a nominal thickness of 32.5 nanometres (λ 0 /4 where λ 0 =500nm) . Further alternate layers 61 and 62 are then deposited followed by a further silicon layer 60. Although not specifically shown in Figure 15, in this example three layers 61 and two layers 62 are provided. The second silicon layer 60 is followed by a first titanium dioxide layer 63 having a nominal thickness of 33.2 nanometres (λ 0 /8 where λ 0 =640nm) . Alternate layers 64 and 65 of silicon dioxide and titanium dioxide having nominal thicknesses of 109.4 nanometres (λ 0 /4 where λ 0 =640nm) and 66.5 (λ 0 /4 where λ 0 =640nm) nanometres, respectively, are then deposited followed by a further titanium dioxide layer 63 and a magnesium fluoride layer 66 having a nominal thickness of 163 nanometres (λ 0 /4 where λ 0 =904nm) . The silicon and silicon dioxide layers 61 and 62 form a first quarter wave stack QSl for light having a wavelength of about 500 nanometres while the silicon -]:oxide and titanium dioxide layers 64 and 65 form a second quarter wave stack QS2 for light having a wavelength of about 640 nanometres. The combination of these two quarter wave stacks QSl and QS2 produces an interference filter structure having the computer simulated transmission characteristic shown in Figure 16. Thus, the silicon/silicon dioxide quarter wavelength stack QSl transmits only light above a wavelength of about 700 nanometres so blocking side bands below 700 nanometres in the transmission characteristics of the titanium dioxide/silicon dioxide quarter wavelength stack QS2 while the titanium dioxide/silicon dioxide quarter wavelength stack QS2 provides a very sharp transition from high reflectance to high transmission at about 750 nanometres and so serves to provide a sharper edge (that is a characteristic which changes more rapidly with wavelength from high reflectance to high transmission) to what would have been a very gradual transition from reflectance to transmission if only the silicon/silicon dioxide quarter wavelength stack QSl had been present. The silicon/silicon dioxide quarter wavelength stack 61,62 is bounded by the layer 60 of silicon which have a thickness of λ/8 at about 500 nanometres. The titanium dioxide/silicon dioxide quarter wavelength stack 64,65 is similarly bounded by titanium dioxide layers 63 which have a thickness equivalent to λ/8 at about 650 nanometres. The final titanium dioxide layer 63 is followed by a magnesium fluoride layer 66. These layers are provided for impedance matching as will be explained below, so as to optimise transmission of light in the desired wavelength range or pass band. Thus, in a manner similar to that discussed above with reference to Figure 5, the quarter wavelength stack layers 61 and 62 together with the bounding silicon layers 60 form a series of Herpin layers with a given Herpin index while the quarter wavelength stack of layers 64 and 65 together with the bounding layers of titanium dioxide 63 form a further series of Herpin layers with a different Herpin index. The magnesium fluoride layer 66 has another refractive index. The arrangement is such that these indices vary gradually from the silicon dioxide passivating layer 8 of the photodiode toward that of air for the wavelength range which the filter is designed to pass or transmit so that undesired reflection by the interference filter structure of light having wavelengths in the pass band of the filter structure is reduced as much as possible. Of course the detailed characteristics of the cut-on filter shown in Figure 16, in particular the wavelength at which the transition from high reflectance to high transmission occurs, may be adjusted by adjusting the thicknesses of the layers 63 to 65 so as to provide a quarter wavelength stack for the desired ' cut-on wavelength and by similarly adjusting the thicknesses of the layers 60 to 62 so as to ensure that the modified side bands resulting from the modified titanium dioxide/silicon dioxide quarter wavelength stack are removed. A photosensitive device having an interference filter structure similar to the interference filter structure 100b shown in Figure 15 may be used in applications where it is desired to detect radiation in the near infra red, for example, in detectors for televisions, video recorders and the like for detecting transmissions from an infra red remote control. Although the above description has been directed to a photosensitive silicon p-i-n diode, it will, of course, be appreciated that the present invention may be applied to other silicon photosensitive pn junction elements such as avalanche photodiodes in which the photosensitive pn junction is formed between relatively highly doped opposite conductivity type regions, pnn+ type devices in which the n+ layer is made thick to bring the n-n+ boundary close to the depletion layer making the device useful for detecting short wavelength, phototransistors or photothyristors and the like. The present invention may be applied to discrete photosensitive devices, to one or two dimensional arrays of photosensitive devices or to so called quadrant detectors comprising a two dimensional array of four detectors suitable for edge tracking, light spot positioning and laser beam alignment and tracking, for example. Where more than one photosensitive device is used, the devices may be individually mounted to a separate substrate or may be integrated in a single semiconductor body. Also, as indicated above, a photosensitive device embodying the invention may be integrated or encapsulated in a hybrid package with semiconductor logic, switching or other control components. It will, of course, be appreciated that conventional optical or other imaging systems, for example ball or fish eye lens, may be used with photosensitive devices embodying the invention. Also, although the silicon photosensitive element described above is formed within a monocrystalline silicon body, the present invention may also be applied to thin film silicon photosensitive elements which may be arranged to, for example, form a two-dimensional array of photosensors. Similarly, although the photosensitive element described above is a vertical semiconductor device, that is the device has its main electrodes on opposed surfaces of the semiconductor body, the present invention may also be applied to lateral semiconductor elements in which the main electrodes are both on the same surface. Also, although the described semiconductor element is produced using a planar process, the present invention may also be applied to so-called mesa semiconductor elements. Also, of course, the conductivity types given above may be reversed. Other modifications and variations will be apparent to those skilled in the art.

Claims: CLAIMS: 1. A photosensitive device comprising a photosensitive element comprising a silicon body having first and second opposite conductivity type regions forming a photosensitive pn junction and a light receiving surface for receiving light to be transmitted to the photosensitive pn junction, the light receiving surface carrying at least one of two electrical contacts coupled to respective ones of the first and second opposite conductivity type regions for extracting charge carriers generated in the vicinity of the pn junction, an interference filter structure deposited on the light receiving surface for allowing only certain wavelengths of light to pass through the light receiving surface and an electrode extending through the interference filter structure to make electrical contact with the at least one electrode. 2. A photosensitive device according to claim 1, wherein the interference filter comprises a Fabry Per r resonator structure comprising at least one Fabry Perot resonator, the Fabry Perot resonator structure being adapted to transmit to the light receiving surface substantially only light having a wavelength in the range between approximately 800 and 900 nanometres. 3. A photosensitive device comprising a silicon photosensitive element having a light receiving surface and an interference filter formed on the light receiving surface, the interference filter comprising a Fabry Perot resonator structure comprising at least one Fabry Perot resonator, the Fabry Perot resonator structure being adapted to transmit to the light receiving surface substantially only light having a wavelength in the range between approximately 800 and approximately 900 nanometres. 4. A photosensitive device according to claim 2 or 3, wherein the at least one Fabry Perot resonator comprises a layer of silicon dioxide having a thickness equal to half a given wavelength provided between layers of silicon having a thickness equivalent to a quarter of the given wavelength, wherein the given wavelength is approximately 830 nanometres. 5. A photosensitive device according to claim 4, wherein the Fabry Perot resonator structure comprises a number of coupled Fabry Perot resonators with adjacent Fabry Perot resonators separated by a layer of silicon dioxide having a thickness equal to approximately a quarter of the given wavelength. 6. A photosensitive device according to claim 2 or 3, wherein the at least one Fabry Perot resonator structure comprises an approximately 286 nanometre thick layer of silicon dioxide provided between two approximately 56 nanometre thick layers of silicon. 7. A photosensitive device according to claim 6, wherein the Fabry Perot resonator structure comprises a number of coupled Fabry Perot resonators with adjacent Fabry Perot resonators separated by an approximately 56 nanometre thick layer of silicon dioxide. 8. A photosensitive device according to claim 1, wherein the interference filter comprises a cut-off filter adapted to inhibit transmission to the light receiving surface of light having a wavelength longer than about 750 nanometres. 9. A photosensitive device comprising a silicon photosensor having a light receiving surface and an interference filter formed on the light receiving surface, the interference filter comprising a cut-off filter adapted to inhibit transmission to the light receiving surface of light having a wavelength longer than about 750 nanometres. 10. A photosensitive device according to claim 8 or 9, wherein the cut-off filter comprises a first stack of alternate layers of titanium dioxide and silicon dioxide having a nominal thickness equivalent to a quarter of a first given wavelength and a second stack of alternate layers of titanium dioxide and silicon dioxide having a nominal thickness equivalent to a quarter of a second given wavelength, with the first and second given wavelengths being approximately 1000 and 816 nanometres, respectively. 11. A photosensitive device according to cl im 10, wherein each of the first and second stacks is bounded by layers of silicon dioxide having a nominal thickness equivalent to one eighth of the first or second given wavelengths, respectively and the first and second stacks are coupled by an impedance matching sequence of alternate layers of silicon dioxide and titanium dioxide. 12. A photosensitive device according to claim 11, wherein the impedance matching sequence comprises first and second layers of silicon dioxide with a nominal thickness equal to an eighth of a third given wavelength separated by a layer of titanium dioxide with a nominal thickness equal to a quarter of the third given wavelength, wherein the given wavelength is approximately 898 nanometres. 13. A photosensitive device according to claim 10 or 12, wherein the first stack comprises alternate layers of silicon dioxide and titanium dioxide having nominal thicknesses of 172.2 and 109.1 nanometres, respectively and the second stack comprises alternate layers of titanium dioxide and silicon dioxide having nominal thicknesses of 89 and 140.4 nanometres, respectively. 14. A photosensitive device according to claim 1, wherein the interference filter structure comprises a cut-on filter structure adapted to allow transmission to the light receiving surface of substantially only light having a wavelength longer than about 700 nanometres. 15. A photosensitive device comprising a silicon photosensor having a light receiving surface and an interference filter formed on the light receiving surface, the interference filter comprising a cut-on filter structure adapted to allow transmission to the light receiving surface of substantially only light having a wavelength longer than about 700 nanometres. 16. A photosensitive device according to claim 14 or 15, wherein the interference filter structure comprises a first stack of alternate layers of silicon and silicon dioxide each having a nominal thickness equivalent to one quarter of a first given wavelength and a second stack of alternate layers of titanium dioxide and silicon dioxide each having a nominal thickness equivalent to one quarter of a second given wavelength with the first and second given wavelengths being approximately 500 and 640 nanometres, respectively. 17. A photosensitive device according to claim 16, wherein the second stack of alternate titanium dioxide and silicon dioxide layers is spaced from the light receiving surface by the first stack. 18. A photosensitive device according to claim 17, wherein the first stack is bounded by layers of silicon having a nominal thickness equivalent to one eighth of the first given wavelength and the second stack is bounded by layers of titanium dioxide having a nominal thickness equivalent to an eighth of the second given wavelength for providing impedance matching through the interference filter structure to the light receiving surface for light which is transmitted by the interference filter structure. 19. A photosensitive device according to claim 18, wherein an impedance matching layer of magnesium fluoride is provided on the final layer of titanium dioxide of the interference filter structure. 20. A daylight sensor for automa ir- 3 ly controlling switching on or off of an outdoor light, comprising a silicon photosensor having a light receiving surface and an interference filter formed on a light receiving surface, the interference filter comprising a cut-off filter structure adapted to inhibit transmission to the light receiving surface of substantially all electromagnetic radiation having a wavelength longer than about 750 nanometres. 21. A daylight sensor for controlling switching on or off of an outdoor light comprising a photosensitive device in accordance with any one of claims 8 to 13. 22. A method of manufacturing a photosensitive device, which comprises providing a silicon body having opposite conductivity type regions forming a photosensitive pn junction, a light receiving surface for receiving light to be transmitted to the photosensitive pn junction carrying at least one of two electrical contacts coupled to respective ones of the first and second opposite conductivity type regions for extracting charge carriers generated in the vicinity of the pn junction, forming an interference filter by depositing a predetermined sequence of thin film layers onto the light receiving surface, and then making electrical contact to the said one electrical contact. 23. A method according to claim 22, which comprises making electrical contact to the said one electrical contact by etching through the interference filter to define a contact hole exposing the electrical contact and then providing electrically conductive material in the contact hole.

Inventors: BISHTON STEPHEN, MANSFIELD DANIEL, PULHAM RICHARD, BISHTON, STEPHEN, MANSFIELD, DANIEL, PULHAM, RICHARD
Applicants: BISHTON STEPHEN, MANSFIELD DANIEL, PULHAM RICHARD, SEMELAB PLC, TAYLOR HOBSON LTD, BISHTON, STEPHEN, MANSFIELD, DANIEL, PULHAM, RICHARD, TAYLOR HOBSON LIMITED
Application year: 1997
Main classification: H01L31
IPC codes: H01L31