WO1990009038A1 - Optical detector - Google Patents

Optical detector Download PDF

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Publication number
WO1990009038A1
WO1990009038A1 PCT/GB1990/000174 GB9000174W WO9009038A1 WO 1990009038 A1 WO1990009038 A1 WO 1990009038A1 GB 9000174 W GB9000174 W GB 9000174W WO 9009038 A1 WO9009038 A1 WO 9009038A1
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WO
WIPO (PCT)
Prior art keywords
filter
detector
silicon
optical
optical detector
Prior art date
Application number
PCT/GB1990/000174
Other languages
French (fr)
Inventor
Simon Ritchie
Paul Charles Spurdens
Mark Drew Learmouth
Original Assignee
British Telecommunications Public Limited Company
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GB898902363A external-priority patent/GB8902363D0/en
Priority claimed from GB898906325A external-priority patent/GB8906325D0/en
Application filed by British Telecommunications Public Limited Company filed Critical British Telecommunications Public Limited Company
Publication of WO1990009038A1 publication Critical patent/WO1990009038A1/en

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/02—Details
    • H01L31/0216—Coatings
    • H01L31/02161—Coatings for devices characterised by at least one potential jump barrier or surface barrier
    • H01L31/02162—Coatings for devices characterised by at least one potential jump barrier or surface barrier for filtering or shielding light, e.g. multicolour filters for photodetectors
    • H01L31/02165—Coatings for devices characterised by at least one potential jump barrier or surface barrier for filtering or shielding light, e.g. multicolour filters for photodetectors using interference filters, e.g. multilayer dielectric filters
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by at least one potential-jump barrier or surface barrier, e.g. phototransistors
    • H01L31/101—Devices sensitive to infrared, visible or ultraviolet radiation
    • H01L31/102—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier
    • H01L31/105—Devices sensitive to infrared, visible or ultraviolet radiation characterised by only one potential barrier or surface barrier the potential barrier being of the PIN type

Definitions

  • This invention relates to optical detectors, in particular to optical detectors incorporating optical filters, and more particularly to such detector and filter combinations suitable for use in optical communications.
  • Optical communications systems employing optical fibres as the transmission medium are increasingly widely used. Such systems are now routinely being incorporated in national and international telecommunications networks.
  • WDM transmitters routinely comprise a plurality of narrow-linewidth semiconductor lasers, the operating wavelengths of the different lasers being suitably spaced apart.
  • WDM demultiplexers or receivers employ either gratings or dielectric transmission filters to separate the various optical wavelengths.
  • a disadvantage of such demultiplexers is that precise alignment of their many components is required, and this, together with the high cost of components such as gratings, makes such demultiplexers unsuitable for mass production, thereby inhibiting the widespread adoption of WDM systems.
  • the present invention seeks to facilitate the mass production of WDM demultiplexers and receivers.
  • an optical detector having an integral wavelength selective filter, the filter comprising a plurality of dielectric layers.
  • a demultiplexer for WDM optical communications system comprising at least two optical detectors each having an integral wavelength selective filter, the wavelength selective filters each comprising a plurality of dielectric layers, and each of said wavelength selective filters having a different pass band.
  • a WDM optical communications system comprising at least one optical fibre, a transmitter for launching WDM optical signals into said fibre, and a receiver arranged to receive said optical signals, wherein the receiver comprises at least two optical detectors each having an integral wavelength selective filter, the wavelength selective filters each comprising a plurality of dielectric layers, and each of said wavelength selective filters having a different pass band.
  • a method of making a tuned optical detector comprising the steps of: forming an optical filter on a surface of a body of semiconductor, the filter comprising a plurality of layers of dielectric material, the refractive indices of the various layers being chosen to provide a desired filter characteristic? and forming a photodetector on a semiconductor substrate; the photodetector and the filter being so arranged that, in use, light passed by the filter is detected by said photodetector.
  • the semiconductor substrate comprises a III-V semiconductor.
  • the filter comprises dielectric layers.
  • the filter may consist essentially of alternate semiconductor and dielectric layers.
  • the alternate semiconductor and dielectric layers may, for example, consist of silicon and silicon monoxide respectively.
  • the photodetector is a PIN photodiode.
  • the PIN photodiode comprises InGaAs/InP.
  • the photodiode is made using metal organic vapour phase epitaxy (MOVPE).
  • Figure 1 shows schematically a substrate entry PIN diode with an integral dielectric filter according to the present invention
  • Figure 2 shows the transmission characteristic of a dielectric filter of the type used in the detector shown
  • Figure 3 is a graph illustrating the quantum efficiency against wavelength for a detector of the type shown in Figure 1;
  • Figure 4 shows schematically various stages in the production of a dielectric filter suitable for use in the present invention.
  • FIG. 1 A preferred embodiment of the invention is shown schematically in Figure 1.
  • the detector is a substrate entry PIN diode, analogous to that described in the paper by M J Robertson et al , Electron. Lett., 1988, Vol. 24,
  • Such a detector comprises an
  • InP substrate 1 an InP buffer layer 2, an IngaAs absorbing layer 3 with an InP capping layer 4.
  • the localised pn junction 5 is made by diffusing a P dopant, for example zinc, through a silicon nitride mask. The sample is then diffused.
  • Metallisation of both the p and n sides 6, 7 is with a Ti/Au bilayer.
  • the filter 7 is a multilayer dielectric filter comprising multiple alternate layers of different dielectrics each of an optical thickness of ⁇ /4 (that is with a thickness of '4n, where n is the refractive index of the layer and ⁇ is the centre wavelength of the pass band), with a spacer layer(s) having an optical thickness of ' 2.
  • the filter 7 may be configured as one or more, typically 3 or 4, Fabry Perot resonators.
  • the filter may consist solely of dielectric materials, or may consist of dielectric and semiconductor materials - for example silicon monoxide and silicon.
  • a preferred filter construction utilises three '2 spacer layers of silicon monoxide each sandwiched between plural alternate '4 layers of silicon and silicon monoxide to provide what is effectively a three Fabry Perot resonator system.
  • Useful performance has been obtained from such a filter having an overall thickness of approximately 7pm.
  • the three Fabry Perot structure had 5 ' 2 silicon monoxide spacer layer, adjacent spacer layers being separated by ten '4 layers.
  • the number of layers and the number and configuration of the spacer layers are chosen to produce a band pass filter exhibiting a high tranmission, narrow linewidth and high peak rejection.
  • a three Fairy Perot structure we have achieved FWHM linewidths of less than 20mn at 1300nm.
  • Four Fabry Perot structures do not provide much narrower linewidth but sharpen the filter profile, reducing the 'tails 1 .
  • a suitable unit cell for a triple Fabry Perot cell comprises the following sequence of lambda/4 layers of silicon (H) and silicon monoxide (L) starting from the substrate side of the filter: HLHLLLLLLHLHL.
  • the sequence LLLLLL of course being the spacer layer.
  • a silicon layer is grown first, starting the above sequence.
  • An alternative sequence suitable for silicon/silicon monoxide in a three or four cavity system is HLHLHLLHLHLH, the LL sequence providing the spacer layer.
  • a further alternative sequence is HLHLLLLHLHL.
  • the filter was deposited in ah electron beam evaporator (a Balzers 640K machine ) with continuous optical monitoring during deposition using a witness plate in the chamber. This allowed the layer deposition conditions to be adjusted during evaporation ( using a well established automatic error compensation technique of the type described in "Thin film optical filters" by H.A MacLeod , published by Adam Hilger,l986 ) for optimum results.
  • the wafer was placed in a resist stripper which removed the stencil and the unwanted filter portions, leaving the patterned filter in place.
  • the pin filter was then metallised on both sides and the metal patterned to produce the final integrated filter pin shown in Figure 1. The process steps are further described below with reference to Figure 4.
  • the filter stack and detector are formed either on opposite faces of the wafer or on the same face.
  • the filter stack is formed using a lift-off technique, an aperture in a bilevel mask layer, with a suitable lip on the upper layer, also defining the site on which the filter stack is to be formed.
  • the significant process steps are illustrated in sequence in Figure 4.
  • the mask base layer 2 in this case polyimide, is spun on.
  • a thin mask upper layer 3 in the case silicon nitride, is deposited on the base layer.
  • a resist 4 typically novolac photoresist, is spun onto the upper mask layer.
  • the resist 4 is patterned, Fig 4d, using a conventional etchant.
  • the silicon nitride layer is then used as a mask as the polyimide is etched, Fig4f, using an oxygen plasma.
  • the polyimide is over etched, with the result that the silicon nitride is undercut.
  • the dielectric filter is formed from silicon and silicon monoxide, Figure 4g, as described earlier.
  • Figure 4h the polyimide base layer is dissolved, releasing the unwanted deposited layers, to leave the filter.
  • An advantageous way of fabricating substrate entry photodiodes according to the invention is to provide the filter structure on the semiconductor wafer before the wafer is processed to form the photodiode structure.
  • the advantage of doing this is that it is possible to check the quality and performance of the filter structure before investing the time and expense necessary to produce the photodiodes. If there are faults in the filter structure it is possible either to reprocess the wafer after stripping off all or some of the filter, or to discard the wafer without losing the high value of a fully processed wafer.
  • Such an " approach depends on having a filter structure which will withstand or which can be made to withstand the processing stages involved in pin photodiode production; silicon/ silicon monoxide can in general be used without the need for further protection.
  • silicon monoxide and silicon are particularly advantageous in that the physical properties of both materials are very suitable: they adhere well to each other and to InP, they are strong , hard, not hygroscopic, and they are compatible with the processing steps used subsequently in device fabrication ( whether or not the subsequent stages comprise the full or partial fabrication of the diode ). Their optical properties are also quite good, and fit well with the wavelength range of principal current interest in the field of optical communications, 1.3 -1.6 urn.
  • Alternatives to silicon monoxide / silicon are silicon dioxide / silicon, silicon nitride / silicon, titanium dioxide / silicon dioxide.
  • Figure 3 is a graph of quantum efficiency against wavelength for a typical substrate entry device, showing a fuul-width-half-maximum (FWHM) linewidth of 23nm, a peak quantum effiiency of 73 per cent at 1234nm and a peak rejection of more than 32dB, which is the limit of our detection system's resolution.
  • FWHM fuul-width-half-maximum
  • Photodetectors incorporating dielectric filters may also be utilised as noise filters in a laser amplifier photodetector configuration to eliminate noise from spontaneous emission and thereby increase the sensitivity obtainable from laser amplifier systems, particularly in 1.3/_cmor 1.55yra optical communications systems.
  • Table 1 sets out some of the more important characteristics of a filter PIN according to the invention and a comparable conventional PIN.

Abstract

An optical detector such as a pin photodiode or avalanche photodiode is provided with an integral dielectric filter. The optical filter preferably comprises a plurality of layers of silicon and silicon monoxide. If the detector is a PIN diode, the filter may be provided on the underside of the substrate before growth of the diode is commenced on the other side.

Description

OPTICAL DETECTOR
This invention relates to optical detectors, in particular to optical detectors incorporating optical filters, and more particularly to such detector and filter combinations suitable for use in optical communications.
Optical communications systems employing optical fibres as the transmission medium are increasingly widely used. Such systems are now routinely being incorporated in national and international telecommunications networks.
There is an .ncreasing desire to employ wavelength division multiplexing (WDH) techniques to increase the useful capacity of optical fibre links. WDM transmitters routinely comprise a plurality of narrow-linewidth semiconductor lasers, the operating wavelengths of the different lasers being suitably spaced apart. WDM demultiplexers or receivers employ either gratings or dielectric transmission filters to separate the various optical wavelengths. A disadvantage of such demultiplexers is that precise alignment of their many components is required, and this, together with the high cost of components such as gratings, makes such demultiplexers unsuitable for mass production, thereby inhibiting the widespread adoption of WDM systems.
The present invention seeks to facilitate the mass production of WDM demultiplexers and receivers.
According to a first aspect of the present invention, there is provided an optical detector having an integral wavelength selective filter, the filter comprising a plurality of dielectric layers. According to a second aspect of the present invention, there is provided a demultiplexer for WDM optical communications system, comprising at least two optical detectors each having an integral wavelength selective filter, the wavelength selective filters each comprising a plurality of dielectric layers, and each of said wavelength selective filters having a different pass band.
According to a third aspect of the present invention, there is provided a WDM optical communications system comprising at least one optical fibre, a transmitter for launching WDM optical signals into said fibre, and a receiver arranged to receive said optical signals, wherein the receiver comprises at least two optical detectors each having an integral wavelength selective filter, the wavelength selective filters each comprising a plurality of dielectric layers, and each of said wavelength selective filters having a different pass band.
According to a fourth aspect of the present invention, there is provided a method of making a tuned optical detector, the method comprising the steps of: forming an optical filter on a surface of a body of semiconductor, the filter comprising a plurality of layers of dielectric material, the refractive indices of the various layers being chosen to provide a desired filter characteristic? and forming a photodetector on a semiconductor substrate; the photodetector and the filter being so arranged that, in use, light passed by the filter is detected by said photodetector.
Preferably, the semiconductor substrate comprises a III-V semiconductor.
In a preferred embodiment, the filter comprises dielectric layers. The filter may consist essentially of alternate semiconductor and dielectric layers. The alternate semiconductor and dielectric layers may, for example, consist of silicon and silicon monoxide respectively. Preferably, the photodetector is a PIN photodiode. Preferably, the PIN photodiode comprises InGaAs/InP. Preferably, the photodiode is made using metal organic vapour phase epitaxy (MOVPE).
Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
Figure 1 shows schematically a substrate entry PIN diode with an integral dielectric filter according to the present invention;
Figure 2 shows the transmission characteristic of a dielectric filter of the type used in the detector shown;
Figure 3 is a graph illustrating the quantum efficiency against wavelength for a detector of the type shown in Figure 1;
Figure 4 shows schematically various stages in the production of a dielectric filter suitable for use in the present invention.
A preferred embodiment of the invention is shown schematically in Figure 1. The detector is a substrate entry PIN diode, analogous to that described in the paper by M J Robertson et al , Electron. Lett., 1988, Vol. 24,
No. 5, pp252 -254, the contents of which are incorporated herein by this reference. Such a detector comprises an
InP substrate 1, an InP buffer layer 2, an IngaAs absorbing layer 3 with an InP capping layer 4. The localised pn junction 5 is made by diffusing a P dopant, for example zinc, through a silicon nitride mask. The sample is then diffused. Metallisation of both the p and n sides 6, 7 is with a Ti/Au bilayer. The active area
2 of the device is low, 55 «m , m order to keep device capacitance low.
To that extent the device is conventional. However, as can be seen from the Figure, to the conventional pin diode there is added a filter. The filter is formed on the substrate so that light which reaches the absorbing region 3 passes first through the filter 7. The filter 7 is a multilayer dielectric filter comprising multiple alternate layers of different dielectrics each of an optical thickness of λ/4 (that is with a thickness of '4n, where n is the refractive index of the layer and λ is the centre wavelength of the pass band), with a spacer layer(s) having an optical thickness of ' 2. The filter 7 may be configured as one or more, typically 3 or 4, Fabry Perot resonators. The filter may consist solely of dielectric materials, or may consist of dielectric and semiconductor materials - for example silicon monoxide and silicon.
. A preferred filter construction utilises three '2 spacer layers of silicon monoxide each sandwiched between plural alternate '4 layers of silicon and silicon monoxide to provide what is effectively a three Fabry Perot resonator system.
Useful performance, as illustrated in Figures 2 and 3, has been obtained from such a filter having an overall thickness of approximately 7pm. The three Fabry Perot structure had 5 ' 2 silicon monoxide spacer layer, adjacent spacer layers being separated by ten '4 layers.
The number of layers and the number and configuration of the spacer layers are chosen to produce a band pass filter exhibiting a high tranmission, narrow linewidth and high peak rejection. With a three Fairy Perot structure we have achieved FWHM linewidths of less than 20mn at 1300nm. Four Fabry Perot structures do not provide much narrower linewidth but sharpen the filter profile, reducing the 'tails1. Of course it is necessary to compromise between reducing the passband's tails - achieved by increasing the number of layers in the filter, and reducing the attenuation caused by the filter - achieved by reducing the overall thickness of the filter.
(A suitable unit cell for a triple Fabry Perot cell comprises the following sequence of lambda/4 layers of silicon (H) and silicon monoxide (L) starting from the substrate side of the filter: HLHLLLLLLHLHL. The sequence LLLLLL of course being the spacer layer. When the filter is grown on the substrate (InP), a silicon layer is grown first, starting the above sequence. Of course the same sequence can be used for four cavity systems and with other materials systems with three or four cavities, but it should not be considered to be optimised for such other systems. An alternative sequence suitable for silicon/silicon monoxide in a three or four cavity system is HLHLHLLHLHLH, the LL sequence providing the spacer layer. A further alternative sequence is HLHLLLLHLHL. )
Other influences affect both these properties. In particular we have found that the use of an electron beam evaporation technique in the ' deposition of the silicon layers and a thermal evaporation technique in the deposition of the silicon monoxide layers are both advantageous. The use of ion beam-assisted deposition is preferred in the deposition of silicon and silicon monoxide as this tends to increase deposit density. The PVD reactor conditions used during the deposition of silicon should be adjusted, as far as is practical, so as to minimise the stress in the silicon layers, because excess stress causes the band edge to spread, increasing absorption, particularly in the 1.3um region.
The filter was deposited in ah electron beam evaporator (a Balzers 640K machine ) with continuous optical monitoring during deposition using a witness plate in the chamber. This allowed the layer deposition conditions to be adjusted during evaporation ( using a well established automatic error compensation technique of the type described in "Thin film optical filters" by H.A MacLeod , published by Adam Hilger,l986 ) for optimum results. We have produced acceptable filters, with greater than 85 per cent transmission and +5nm centre wavelength tolerance, when the errors in the optical thickness (which is of course dependent on both thickness and refractive index ) are controlled to +0.625. After the deposition, the wafer was placed in a resist stripper which removed the stencil and the unwanted filter portions, leaving the patterned filter in place. The pin filter was then metallised on both sides and the metal patterned to produce the final integrated filter pin shown in Figure 1. The process steps are further described below with reference to Figure 4.
The filter stack and detector are formed either on opposite faces of the wafer or on the same face. The filter stack is formed using a lift-off technique, an aperture in a bilevel mask layer, with a suitable lip on the upper layer, also defining the site on which the filter stack is to be formed. The significant process steps are illustrated in sequence in Figure 4. First, the mask base layer 2, in this case polyimide, is spun on. Following any necessary curing or hardening of the base layer 2, a thin mask upper layer 3, in the case silicon nitride, is deposited on the base layer. Next, Figure 4c, a resist 4, typically novolac photoresist, is spun onto the upper mask layer. The resist 4 is patterned, Fig 4d, using a conventional etchant. The silicon nitride layer is then used as a mask as the polyimide is etched, Fig4f, using an oxygen plasma. The polyimide is over etched, with the result that the silicon nitride is undercut. The dielectric filter is formed from silicon and silicon monoxide, Figure 4g, as described earlier. Finally, Figure 4h, the polyimide base layer is dissolved, releasing the unwanted deposited layers, to leave the filter.
A generally suitable lift-off technique is also described in the paper by P Grabbe et al, in J. Vac. Sci. Tech., Vol. 21, Part I, 1982, pp 33-35, the contents of which are herein incorporated by this reference. Following the multiple deposition steps, the mask, which is typically silicon nitride over polyimide, is removed, along with the excess silicon and silicon monoxide. This technique makes possible the reproducible production of relatively cheap, wavelength selective, photodiodes for use as simple demultiplexers.
An advantageous way of fabricating substrate entry photodiodes according to the invention is to provide the filter structure on the semiconductor wafer before the wafer is processed to form the photodiode structure. The advantage of doing this is that it is possible to check the quality and performance of the filter structure before investing the time and expense necessary to produce the photodiodes. If there are faults in the filter structure it is possible either to reprocess the wafer after stripping off all or some of the filter, or to discard the wafer without losing the high value of a fully processed wafer. Such an "approach depends on having a filter structure which will withstand or which can be made to withstand the processing stages involved in pin photodiode production; silicon/ silicon monoxide can in general be used without the need for further protection.
The use of the combination of silicon monoxide and silicon is particularly advantageous in that the physical properties of both materials are very suitable: they adhere well to each other and to InP, they are strong , hard, not hygroscopic, and they are compatible with the processing steps used subsequently in device fabrication ( whether or not the subsequent stages comprise the full or partial fabrication of the diode ). Their optical properties are also quite good, and fit well with the wavelength range of principal current interest in the field of optical communications, 1.3 -1.6 urn. Alternatives to silicon monoxide / silicon are silicon dioxide / silicon, silicon nitride / silicon, titanium dioxide / silicon dioxide.
The current- voltage characteristics of these substrate entry devices are similar to those from the same size pin detectors without a filter deposited, demonstrating that the filter deposition stage does not significantly affect the pin photodiode pn junction on the other side of the substrate. To confirm that the filter patterning stage did not significantly affect the filter characteristics, the transmission characteristics of a filter patterned as above on a blank InP substrate have been compared with an unpatterned test piece. There was no significant difference. Figure 3 is a graph of quantum efficiency against wavelength for a typical substrate entry device, showing a fuul-width-half-maximum (FWHM) linewidth of 23nm, a peak quantum effiiency of 73 per cent at 1234nm and a peak rejection of more than 32dB, which is the limit of our detection system's resolution.
In order to assess the stability of the dielectric films, the transmission spectra of patterned filters on InP substrates were measured before and after being immersed in boiling water for 1 hour. There was no discernible change. Samples were also heated,in air,to 300C without a change in room temperature characteristics.
Photodetectors incorporating dielectric filters may also be utilised as noise filters in a laser amplifier photodetector configuration to eliminate noise from spontaneous emission and thereby increase the sensitivity obtainable from laser amplifier systems, particularly in 1.3/_cmor 1.55yra optical communications systems.
Devices using silicon/silicon monoxide filters have exhibited filter bandwidths of less than 20nm with peak transmission approaching 100Δ. Such filters are usable in the wavelength range 1.3 to 1.6μm.
Table 1 sets out some of the more important characteristics of a filter PIN according to the invention and a comparable conventional PIN.

Claims

An optical detector having an integral wavelength selective filter, the filter comprising a plurality of dielectric layers.
An optical detector as claimed in claim 1 wherein the dielectric layers each have a thickness substantialy equal to lambda/ ( 4n ), wherein: lambda is the the centre wavelength of the passband of the filter; and n is the refractive index of the respective layer at that wavelength.
An optical detector as claimed in claim 1 or claim 2 wherein a plurality of said layers consist of silicon.
An optical detector as claimed in claim 3 wherein others of said plurality of dielectric layers consist of silicon monoxide.
An optical detector as claimed in any one of the preceding claims wherein the detector is a PIN photodiode.
An optical detector as claimed in claim 5 wherein the detector is a substrate entry device.
An optical detector as claimed in any one of claims 1 to 4 wherein the detector is an avalanche photodiode.
An optical detector according to any one of thepreceding claims wherein the centre wavelength of the filter passband is between 1.3 and 1.6 microns inclusive. 9 A wavelength division multiplexed optical communications system comprising at least two detectors according to any one of the preceding claims, the wavelength selective filter on two or more of said detectors having passbands whose centre wavelengths differ by 20nm or more.
10 A method of making a detector as claimed in claim 1, which method comprises the steps of forming an optical filter on a body of semiconductor, the filter comprising a plurality of layers of dielectric material.
11 A method as claimed in claim 10 wherein the method includes the step of forming a PIN photodiode on said body after the formation of said filter.
12 A method as claimed in claim 10 or claim 11 wherein the method comprises a lift-off step in which a thickness of 7 microns or more is removed.
13 A method as claimed in claim 12 wherein the thickness removed is between 7 and 10 microns inclusive.
14 A method as claimed in claim 12 or claim 13 wherein a multilayered structure of silicon and silicon monoxide having a thickness of 7 microns or more is removed in said lift-off step.
PCT/GB1990/000174 1989-02-03 1990-02-05 Optical detector WO1990009038A1 (en)

Applications Claiming Priority (4)

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GB8902363.4 1989-02-03
GB898902363A GB8902363D0 (en) 1989-02-03 1989-02-03 Optical detector
GB8906325.9 1989-03-20
GB898906325A GB8906325D0 (en) 1989-03-20 1989-03-20 Optical detector

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US5216237A (en) 1993-06-01
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