GB2366872A

GB2366872A - Improvemnets in and relating to optoelectric devices

Info

Publication number: GB2366872A
Application number: GB0108265A
Authority: GB
Inventors: Simon Eric Hicks; Colin Roy Stanley; John Haig Marsh
Original assignee: University of Glasgow
Current assignee: University of Glasgow
Priority date: 2000-04-03
Filing date: 2001-04-03
Publication date: 2002-03-20
Also published as: GB0008126D0; GB0108265D0

Abstract

There is disclosed an improved optoelectronic device (5a) and a process for fabricating same. The optoelectronic device (5a) includes at least one optoelectronic device element (10a), a portion of which intimately contacts a surface of a substrate (110a), the substrate (100a) being at least partially transparent, the surface being at least partially electrically conductive.

Description

<Desc/Clms Page number 1> IMPROVEMENTS IN AND RELATING TO OPTOELECTRONIC DEVICES FIELD OF INVENTION This invention relates to an improved optoelectronic device and to a process for fabricating same. The invention particularly, though not exclusively, relates to an improved optoelectronic device providing an array of optoelectronic device elements such as optical modulators. BACKGROUND OF INVENTION Presently, a starting point for fabrication of so- called 'smart pixel' devices is a III-V substrate having a p-contact layer, an MQW structure and an n-contact layer, making a p-i-n diode, grown sequentially onto a substrate. Mesa structures are then fabricated to isolate each pixel, and metal contacts are fabricated onto the p and n-layers.

111e 11-1aye1., W111 C.LL11er LCC@U1t@ d ULY CLUll SLaye UL a11 lUil implantation stage to realise the required electrical contact. A typical example is given in K_W_ Goossen, J.A. Walker, L.A. D'Asaro, S.P. Hui, B. Tseng, R. Leibenguth, D. Kossives, D.D. Bacon, D. Dahringer, L.M.F. Chirovsky, A.L, Lentine, D.A.B, Miller, IEEE Photonics Technology Letters, Vol 7, No 4, p360-362, 1995. An important feature to note in the above design is that the n and p-metal pads are on the same face of the device/sample.

Solder-bond technology is then used to bond the n and p-contacts to corresponding contacts on a silicon wafer containing the relevant drive circuitry. The III-V

substrate is then removed to allow optical access to the diode structures. An antireflection (AR) coating is often applied. Each pixel, therefore, requires two solder bump bonds on the same face and, therefore, needs to be more than twice the area of each solder bond. Also an optically active area of each pixel corresponds only to an area above the n-contact, i.e. the area above the p-contact is socalled-'deadspace'. This means that the total optically active pixel area is less than half the III-V chip area. A prior art device structure is shown in Figure 1.

OBJECTS OF INVENTION It is an object of one or more aspects of the present invention to obviate or at least mitigate one or more of the aforementioned problems in the prior art.

Further objects of embodiments of various aspects of the present invention include: provision of a simplified III-V fabrication process involving less lithography, metallisation and implantation stages resulting in lower fabrications costs and higher yeilds as compared to the prior art; provision for each pixel of an increased percentage of optically active area, i.e. decreased 'deadspace` compared to the prior art; the ability to fabricate pixels of less than half the size of the prior art (assuming a given bump bond size), so giving, for example, at least four times as many pixels in

a given area; provision of a transparent substrate which acts as a common contact and also as a robust encapsulant.

SUMMARY OF INVENTION According to a first aspect of the present invention there is provided an optoelectronic device including at least one optoelectronic device element a portion of which intimately contacts a surface of a substrate, the substrate being at least partially transparent, the surface being at least partially electrically conductive.

It will be understood that by "optically transparent" it is meant that the substrate allows passage of optical radiation at a wavelength of operation of the optoelectronic device element(s).

By this arrangement the/each optoelectronic device element may be both optically and electrically addressed, in usePreferably, the substrate comprises a substrate member and a surface layer, the surface layer providing the surface.

Preferably, the surface layer is electrically conductive and optically transparentPreferably, the surface layer is formed at least partly from Indium Tin Oxide (ITO).

Preferably, the substrate member is electrically non- conductive.

Preferably, the substrate member is formed from an optically transparent material such as a glass.

Preferably, the optoelectronic device comprises a plurality of individually electrically and optically addressable device elements.

In one embodiment one or more of the optoelectronic device elements comprise an optical modulator.

Preferably, in said one embodiment the/each optical modulator is a Multiple Quantum Well (MQW) modulator. Preferably, another portion of the at least one optoelectronic device element is in electrical communication with device drive circuitry provided on a further substrate.

Preferably, the at least one optoelectronic device element is provided between the substrate and the further substrateThe at least one optoelectronic device element may be formed from a Gallium Arsenide (GaAs) based materials system.

According to a second aspect of the present invention there is provided an apparatus including an optoelectronic device, the optoelectronic device including at least one optoelectronic device element a portion of which intimately contacts a surface of a substrate, the substrate being at least partially transparent, the surface being at least partially electrically conductive.

The apparatus may be an optical communications system,

an optical computer, or the like.

According to a third aspect of the present invention there is provided a process for fabricating an optoelectronic device, the optoelectronic device including at least one optoelectronic device element a portion of which intimately contacts a surface of a substrate, the substrate being at least partially transparent, the surface being at least partially electrically conductive, the fabrication process including the steps of: (a) forming the at least one optoelectronic device element; (b) retaining the portion of the/each optoelectronic device element in intimate contact with the surface of the substrate.

Preferably, the fabrication process includes the steps of: (c) retaining another portion of the/each optoelectronic device element in intimate contact with a further substrate which may carry drive circuitry for the/each optoelectronic device element.

Preferably, the fabrication process includes the step of (d) forming an electrical contact between the surface of the substrate and the further substrate. Preferably, the substrate may be provided by the steps of

providing a substrate member; forming a surface layer on the substrate member so as to provide the surface of the substrate.

The surface layer may be formed on the substrate member by evaporation or spinning or the like.

BRIEF DESCRIPTION OF DRAWINGS An embodiment of the present invention will now be described, by way of example only, and with reference to the accompanying drawings which are: Figure 1 a schematic side view of an optoelectronic device providing an array of optical modulators according to the prior art; Figure 2 a series of schematic diagrams showing steps (a - o) involved in fabricating the device of Figure 1; Figure 3 a schematic side view of an optoelectronic device providing an array of optical modulators according to an embodiment of the present invention to the same scale as Figure 1; Figure 4 a series of schematic diagrams showing (a)-(i) steps involved in fabricating the device of Figure 3; and Figure 5 a schematic illustration of an apparatus

including the device of Fig. 3. DETAILED DESCRIPTION OF DRAWINGS Referring initially to Fig. 1, there is shown a prior art optoelectronic device, generally designated 5, including at least one optoelectronic device element 10_ In this case the device 5 includes two optoelectronic device elements 10 in the form of Multi Quantum Well (MQW) optical modulators, each of which may be both optically and electrically addressed, in use.

Each optoelectronic device element 10 comprises a player 15, an MQW region 20, an ion implanted region 25, an n-layer 30, and first and second contact metallisation regions 35;40.

Each of the first and second contact metalisation regions 35;40 is electrically connected to silicon drive circuitry 45 via a respective first or second solder bump- bond 50;55_ Each of the optoelectronic device elements 10 is potted within an epoxy 60. Further, an anti-reflective (AR) coating 65 is provided on a surface of the player 15 remote from the silicon drive circuitry 45.

It can be seen from Fig. 1 that each optoelectronic device element 10 includes an optically active region 70 which is around 50 s or even less of the width of the optoelectronic device element 10. The optoelectronic device element 10 may be addressed by optical radiation as

indicated in Fig_ 1 by arrows A. Further, each optoelectronic device element 10 may be electrically addressed via the silicon drive circuitry 45 via a path from first solder bump-bond 50 first contact metalisation region 35, n-layer 30, ion implanted region 25, passing through the MQW region 20, player 15 thence through the MQW region 20, n-layer 30 and second contact metalisation region 40 and finally through second solder bump-bond 55 returning to silicon drive circuitry 45.

An optoelectronic device 5 according to the prior art is fabricated by a process illustrated in Figs. 2(a) to (o). Referring to each of these steps in sequence, the process steps are as follows.

A substrate 65 such as Gallium Arsenide (GaAs) - is provided. Adjacent to a surface of the substrate 65, there is provided an etch stop 70 and adjacent a surface of the etch stop 70 is provided the n-layer 30. Adjacent a surface of the n-layer 30 is provided the MQW region 20 and adjacent a surface of the MQW region 20 is provided the player 15. A resist layer 75 may be spun onto a surface of the player 15 and a mask (not shown) employed to open windows 80 in the resist layer 75 via standard optical lithography techniques. Referring to Fig. 2(c) the ion implanted regions 25 are formed by ion implantation into the n-layer 30.

Referring to Figure 2 (d), a further resist layer 85 is spun onto the player 15 and windows 90 opened in the further resist layer 85 by standard lithography techniques. Referring to Fig. 2(e) the optoelectronic device elements 10 may be formed as mesa structures 95 by wet or dry etching through the various layers to the etch stop 70.

In Figure 2 (f) a yet further resist layer 96 is spun onto the player 15 and windows 97 opened therein. Referring to Fig. 2(g) the player 15 is etched to isolate the n-layer 30 from the player 15 electrically.

Referring to Figure 2(h) a still further resist layer 98 is spun onto the mesa structures 95 and windows 99 opened therein. The first and second solder bump-bonds 50 and 55 are thereafter deposited on each optoelectronic device element 10, and the resist layer removed by standard techniques. The substrate 65 may thereafter be sawed into individual modulator arrays and "flip chip bonded" to silicon drive circuitry 45 which may be implemented in CMOS. This is shown in Fig. 2(k). Referring to Fig. 2(1), epoxy 60 may thereafter be applied, and referring to Fig. 2(m) the substrate 65 removed. Referring to Fig. 2(n) an anti-reflective (AR) coating 65 may thereafter be applied and as shown in Fig. 2(o) the assembly may then be sawed into individual optoelectronic device element 10 arrays and an external bond contact 94 made thereto.

Referring now to Figure 3 there is shown an optoelectronic device, generally designated 5a, according

to an embodiment of the present invention. The optoelectronic device 5a includes a plurality of optoelectronic device elements 10a - in this embodiment four elements 10a. As can be seen from Fig_ 3, a portion of each optoelectronic device element 10a intimately contacts a surface of a substrate 100a, the substrate 100a being at least partially transparent, the surface being at least partially electrically conductive- By this arrangement the/each optoelectronic device element 10a may be both optically and electrically addressed in use.

Each optoelectronic device element comprises an n- layer 30a adjacent a Multiple Quantum Well (MQW) region 20a which is adjacent a player 15a. The player 15a of each optoelectronic device element 10a is in contact with a contact metallisation region 35a which is in electrical contact with a solder bump-bond 50a. Each solder bump-bond 50a electrically connects the respective optoelectronic device element 10a to silicon drive circuitry 45a.

As can also be seen from Fig_ 3 each n-layer 30a intimately contacts an electrically conductive and optically transparent surface layer 105a eg. an Indium Tin Oxide (ITO) layer) provided on the substrate 100a; the substrate in this embodiment being made of a glass. Also, on a surface of the substrate 100a remote from the surface layer 105a, there may beneficially be provided an anti- reflective (AR) coating 65a.

By the above described arrangement each optoelectronic

device elements 10a may be optically addressed as can be seen from arrows "B" indicating optical radiation, and may also be electrically addressed via the Silicon drive circuitry 45a through the solder bump-bond 50a, contact metallisation 35a, player 15a, MQW region 20a, n-layer 30a and electrically conductive and optically transparent surface layer 105a. A comparison of Fig. 3 and Fig. 1 clearly shows that a higher density of optoelectronic device elements 10a may be provided for any given unit area in the invention as compared to the prior art design.

In this embodiment each optoelectronic device element 10a comprises an optical modulator. However, it will be appreciated by those skilled in the art that the present invention is not necessarily limited to the optoelectronic device elements 10a being optical modulators. The optical optoelectronic device element 10a may indeed be any type of optoelectronic device element requiring both optical and/or electrical addressing.

A process for fabrication of an optoelectronic device 5a according to the present invention will now be described, with reference to Fig. 4(a) to (i) . Firstly, there is provided a semiconductor substrate 65a which may be, for example, a Gallium Arsenide (GaAs) substrate. Referring to Fig. 4(a) on a surface of the substrate there is provided an etch stop 70a and on a surface of the etch stop there is provided the player 15a. Further, on a

surface of the player 15a there is provided the MQW region 20a, while on a surface of the MQW region 20a there is provided the n-layer 30a. Referring to Figure 4(b) a plurality of mesa structures 95 may be formed on the substrate 65a by conventional optical lithography and dry or wet etching techniques through the player 15a to the etch stop 70a.

Referring to Figure 4(c) the substrate may then be flip-chip bonded onto the at least partially transparent substrate 100a_ As mentioned above the at least partially transparent substrate may be formed from a glass material having a surface layer 105a formed thereon. The surface layer 105a in this embodiment is formed of indium tin oxide (ITO). The surface layer 105a is therefore both optically transparent and electrically conductive. Two techniques may be employed to provide the surface layer 105a on the substrate 100a; thermal bonding or liquid - ITO bonding. Firstly, considering thermal bonding. The glass (or similar transparent material) substrate is coated with a layer of transparent and conductive indium tin oxide (ITO) by standard fabrication techniques. The substrate 100a is then placed over the mesa structures 95a (with the surface layer 105a adjacent the mesa structures 95a. Pressure may then be applied at an elevated temperature of around say 300`C to fuse the surface layer 105a to the n-layer 30a which comprises a contact layer of each optoelectronic device element 10a. Each optoelectronic device element 10a

may be conveniently referred to as a "pixel".

The second bonding technique known as liquid - ITO bonding is as follows. The glass (or similar transparent material) substrate 100a may be coated with a layer of liquid - ITO, i.e. the ITO constituents in a solvent. The coated substrate 100a is then brought into contact with the mesas structures 95a and heated to remove the solvent and form an ITO surface layer 105a. This surface layer 105a acts as an adhesive and as a transparent conductor.

Referring now to Fig. 4(d), the III-V semiconductor substrate 65a may be removed by convention techniques. Referring to Fig. 4(e) a photo-resist layer 85a may be spun onto the mesas structures 95a and windows 90a opened up therein by conventional optical mithogrothy techniques. Referring to Fig. 4(f) Gold (Au) and then Indium (In) metallisation may be evaporated to form the bump-bonds 50a. Further a separate bump-bond 110a is formed on the surface layer 105a so as to ensure a closed circuit, as will become apparent hereinafter. Referring to Fig. 4(g), the substrate 65a may then be sawed into individual optoelectronic devices 5a. Referring to Fig. 4(h) the individual optoelectronic device 5a may then be flip-chip bonded onto a silicon drive circuitry 45a which may comprise, for example, a six inch wafer and be implemented in CMOS technology. The individual devices 5a may then be sawed up to release individual completed devices 5a.

It will be appreciated that although the disclosed

embodiment given hereinabove is implemented in gallium arsenide that devices according to the present invention may be implemented in other III-V semiconductor materials systems, whether binary, tertiary, quaternary or otherwise.

Referring to Fig. 5 there is shown an apparatus, generally designated 200a, including a plurality of optoelectronic devices 5a according to the present invention. For ease of reference three such optoelectronic devices 5a are shown in Fig. 5, the devices 5a being identified as X, Y and Z respectively. The optoelectronic devices 5a are arranged behind a retro-reflective lens system, generally designated 205a, as is known in the art. As can be clearly seen from Fig. 5, optical radiation input into the retro-reflective lens system 205a at a given angle is selected to be output therefrom at a predetermined angle such that radiation input into the retro-reflective lens system 205a at a given angle is "seen" by a particular optoelectronic device 5a. Further optical radiation output from a particular optoelectronic device element 5a towards the retro-reflective lens system 205a is output from the retro-reflective lens systems 205a along a predetermined path. Thus, it can be seen that use of the retro- reflective lens system 205a allows one to optically input to and optically output from each optoelectronic device 5a along a predetermined path. Such an arrangement could be of use in local area networking or in optical computers.

In the embodiment hereinbefore described it can be

seen that firstly, the "diode" structure on the III-V substrate is grown. Mesa structures are then fabricated by etching right through the p-i-n structure. A transparent substrate (typically a glass) with a transparent yet electrically conductive surface layer (typically Indium Tin Oxide (ITO)) is bonded to the mesas. The III-V substrate is then removed leaving individual modulator pixels attached to the transparent substrate.

Lithography is then used to open windows in a resist above each pixel (and wherever else they are required) - Au evaporation followed by In evaporation is then performed and lift-off is used to leave Au/In bump-bond pads covering each pixel. This is then flip-chip bonded to a silicon chip with the relevant drive circuitry and corresponding bump-bond pads.

The transparent and conductive substrate acts as a robust encapsulant and supplies a common contact to one side of the pixels- The Silicon circuit addresses each pixel independently from the other side. Optical access to the diodes is through the transparent substrate with the Au layer acting as a mirror. Note that the percentage area of each pixel that is optically active approaches 1000. Also, the pixels can be placed very close together (limited only by lithographic and etching processes) and therefore the 'deadspace' is far reduced.

It will be appreciated that the embodiments of the present invention hereinbefore described are given by way

of example only, and are not meant to limit the scope thereof in any way.

In particular, it will be appreciated that although the disclosed embodiment relates to a novel design and corresponding fabrication process for arrays of optical modulators, wherein the device uses electrical signals to independently modulate an array of independent Multi Quantum Well (MQW) modulators, other optoelectronic devices requiring both electrical and optical addressing fall within the scope of the invention.

Further, although Au/In bump-bonds are suggested hereinabove, any suitable solder bond technology could be used.

Also, hereinbefore there is disclosed an array of detectors a11 having a common contact which enables use of an unpatterned continuous layer of ITO. However, one could also address each individual pixel with a different voltage from the ITO side by suitable patterning of the ITO layer. This would typically be in the form of (but not limited to) individual tracks or a grid structure.

Also an anti-reflection (AR) coating can be deposited on a surface of the transparent substrate remote from the optoelectronic device elements, i.e. at the final substrate/air interface. This will tend to decrease the amount of incident light reflected (which by definition cannot be modulated) at the substrate/air interface.

Further, an MQW mirror structure could be incorporated

in the original III-V wafer growth to further increase the reflectance level.

Finally, the thickness of all the layers within the final device (including the III-V layers and the ITO layer) can be sought to be optimised so as to seek to give an optimum reflection modulation performance.

Claims

CLAIMS 1. An optoelectronic device including at least one optoelectronic device element a portion of which intimately contacts a surface of a substrate, the substrate being at least partially transparent, the surface being at least partially electrically conductive. 2. An optoelectronic device as claimed in Claim 1, wherein the substrate comprises a substrate member and a surface layer, the surface layer providing the surface. 3. An optoelectronic device as claimed in Claim 2, wherein the surface layer is electrically conductive and optically transparent- 4. An optoelectronic device as claimed in either of Claims 1 or 2, wherein the surface layer is formed at least partly from Indium Tin Oxide (ITO). 5. An optoelectronic device as claimed in Claims 2 to 4, wherein the substrate member is electrically nonconductive. 6. An optoelectronic device as claimed in Claims 2 to 5, wherein the substrate member is formed from an optically transparent material.

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7. An optoelectronic device as claimed in Claim 1, wherein the optically transparent material is a glass. 8. An optoelectronic device as claimed in Claim 1, wherein the optoelectronic device comprises a plurality of individually electrically and optically addressable device elements. 9. An optoelectronic device as claimed in any preceding claim, wherein the or at least one of the optoelectronic device elements comprises an optical modulator. 10. An optoelectronic device as claimed in Claim 9, wherein the or the at least one optical modulator is a Multiple Quantum Well (MQW) modulator. 11_ An optoelectronic device as claimed in any preceding claim, wherein another portion of the at least one optoelectronic device element is in electrical communication with device drive circuitry provided on a further substrate. 12. An optoelectronic device as claimed in Claim 11, wherein the at least one optoelectronic device element is provided between the substrate and the further substrate.

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13. An optoelectronic device as claimed in any preceding claim, wherein the at least one optoelectronic device element is formed from a Gallium Arsenide (GaAs) based materials system. 14. An apparatus including an optoelectronic device, the optoelectronic device including at least one optoelectronic device element a portion of which intimately contacts a surface of a substrate, the substrate being at least partially transparent, the surface being at least partially electrically conductive. 15. An apparatus as claimed in Claim 14, wherein the apparatus is selected from an optical communication system, or an optical computer. 16. A process of fabricating an optoelectronic device, the optoelectronic device including at least one optoelectronic device element a portion of which intimately contacts a surface of a substrate, the substrate being at least partially transparent, the surface being at least partially electrically conductive, the fabrication process including the steps of: (a) forming the at least one optoelectronic device element; (b) retaining the portion of the/each optoelectronic device element in contact with the surface of the

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substrate. 17. A process of fabricating an optoelectronic device as claimed in Claim 16, wherein the fabrication process includes the step either before or after step (1b) of: (c), retaining another portion of the/each optoelectronic device element in intimate contact with a further substrate. 18. A process of fabricating an optoelectronic device as claimed in Claim 17, wherein the fabrication process includes the step of: (d) forming an electrical contact between the surface of the substrate and the further substrate. 19. A process of fabricating an. optoelectronic device as claimed in any of Claims 16 to 18, wherein the substrate is provided by the steps of: providing a substrate member; forming a surface layer on the substrate member so as to provide the surface of the substrate. 20. A process of fabricating an optoelectronic device as claimed in Claim 19, wherein the surface layer is formed on the substrate member by evaporation or spinning. 21. An optoelectronic device as hereinbefore described

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with reference to Figure 3. 22. An apparatus including an optoelectronic device as hereinbefore described with reference to Figure 5. 23. A process of fabricating an optoelectronic device as hereinbef ore described with reference to Figures 4(a) to 4(i).