Abstract:
An optical device, such as an electro-absorption modulator, has a waveguide formed onto a base. Electrodes adjacent the waveguide are used to selectively apply an electric field to the device in order to control the passage of light through the device. To prevent overheating of the device in a region near the point of optical entry, the electrode, which acts as a thermal conduit to dissipate heat from the waveguide is extended over the region. To prevent undesirable electrical contact between the electrode and the region, the normally electrically conducting contact layer between the electrode and the waveguide is removed in that region and replaced by an isolation dielectric.

Description:
FIELD OF THE INVENTION 
     The present invention relates to reduction of local heating in an optical device, and particularly reduction of local heating by using a thermally conducting electrode. 
     BACKGROUND OF THE INVENTION 
     Devices such as lasers, optical modulators and photo detectors experience heating due to the nature of the physics involved in their operation. 
     An example in which local heating is problematic occurs in optical modulator devices. An optical modulator such as an electro-absorption modulator includes a waveguide through which light is directed. Light is normally allowed to pass through the device. An electrode on top of the waveguide is used in conjunction with another electrode at the bottom of the device to introduce an electric field in the waveguide. This electric field changes the semiconductor properties so that light passing through the waveguide is absorbed. This absorption of the light results in heating of the device. Overheating of the device will cause breakdown of one or more components and result in failure of the device. 
     The electrode itself contributes to dissipation of heat in the device. In particular, in a region of the waveguide where the electrode makes contact with the waveguide, the electrode acts as a thermal conductor and creates a thermal path which can carry heat away from that portion of the waveguide thereby reducing the possibility of overheating. However, the electrode does not necessarily extend the entire length of the waveguide as this can have undesirable effects such as increasing capacitance, thereby decreasing the speed at which the modulator reacts. For example, the electrode often does not extend to the point of optical entry of light into the device. In areas not covered by the electrode there is reduced thermal conduction. As a result, heat is not dissipated as efficiently and local overheating of the waveguide in such an area can result in catastrophic failure of the device. 
     Accordingly, such devices must be used within certain operating limits or constraints such as limiting the amount of optical power that can be delivered in the device, or limiting the amount of electrical bias which can be applied. These constraints can limit the range of applications to which the device can be put, particularly in evolving high power optical networks. 
     Another problem which exists in such devices is the termination of the electrode. For example, in an InGaAs electro-absorption modulator the electrode can be formed of several layers of metal, including a gold layer. Abrupt termination of the electrode can permit gold to diffuse into the device, rendering it inoperative. Therefore, electrodes in such devices are typically raised at their termination from the semiconductor layer. Although techniques for terminating the electrode in a raised fashion away from the contact layer of the device prevent such diffusion, the requirement to do so is undesirable and increases the complexity of the manufacturing process. A further disadvantage of this termination technique is that the raised portion of the electrode does not serve to conduct heat away from the device. 
     Overheating also exists in lasers such as the 980 nm pump laser. The 980 nm pump laser is a semiconductor laser having a waveguide which terminates in an exposed cross-section, known as a facet. In particular, overheating is a problem in the vicinity of the facet because the facet forms an interface with the air. A technique to mitigate the interface problem is to specially alter a portion of the waveguide near the facet so that it is not energized or “pumped” and does not act as a laser. The modified portion of the waveguide, for example the first 100 microns of the waveguide from the facet, is known as a window. However, even with this approach, local overheating may still be a problem. Similar to the example of the optical modulator, if an electrode stripe terminates before the facet region, then that region will not does not have an efficient thermal path to conduct heat away. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a device and method to overcome or mitigate at least one of the disadvantages of previous methods and devices for reducing localized heating in optical devices. 
     According to an aspect of the present invention, a device for reducing localized heating in an optical waveguide includes an electrode for applying an electrical bias across the optical waveguide and a contact layer for providing an interface or common boundary between the electrode and the optical waveguide. The contact layer has an electrically and thermally conductive first region, and a thermally and non-electrically conductive second region for dissipating heat in a region of localized heating. 
     According to another aspect of the present invention, the thermally and non-electrically conductive second region, as discussed above, includes an isolation dielectric, such as SiN. The electrode terminates in a terminal end which is in abutting contact with the second region. 
     According to a further aspect of the present invention, a device for reducing localized heating in an optical waveguide includes a contact layer on the optical waveguide. The contact layer which extends longitudinally along a portion of the waveguide has an electrically and thermally conductive first region, and a thermally and non-electrically conductive second region for dissipating heat in a region of localized heating. A thermally conducting electrode extends longitudinally along the contact layer. 
     According to a still further aspect of the present invention, there is provided a method for reducing localized heating in a device through which light passes. The device includes a waveguide on a semiconductor substrate. The method includes providing an electrode adjacent the waveguide for applying an electric field. A thermally conducting and electrically conducting region is provided between the waveguide and the electrode in a region of desired electrical contact between the electrode and the waveguide while a thermally conducting and electrically insulating region is provided between the waveguide and the electrode outside the region of desired electrical contact between the electrode and the waveguide. 
     According to a still further aspect of the present invention, a method of manufacturing a device for controlling light which passes through the device is provided. The method includes providing an optical waveguide on a semiconductor substrate and adding to the surface of the waveguide a contact layer of electrically conducting and thermally conducting material. The contact layer is then masked to define a first region of the contact layer and leaving exposed a second region of the contact layer. The electrically conducting material is then etched away in the exposed region of the contact layer. A blanket deposit of an electrically insulating and thermally conducting material is added to the first region and the etched away second region of the contact layer. Then the second region is masked leaving exposed at least a part of the first region of the contact layer. The electrically insulating and thermally conducting material is etched away in the exposed part of the first region of the contact layer. Metal is then deposited onto the contact layer to form an electrode. 
     Reduction of local heating according to the present invention permits greater optical power to be delivered to the device. Another advantage is that for a given level of optical input, a higher reverse bias voltage can be used to modulate the light signal thereby permitting increased speed of the device. Accordingly, the device can be used in a greater variety of situations or for a greater variety of purposes as the previously existing operating constraints have been lessened. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be further described, by way of example, with reference to the accompanying drawings in which: 
     FIG. 1 is a section view illustrating an optical modulator; 
     FIG. 2 is a side view of an electrode of the device of FIG. 1; 
     FIG. 3 is a side view of an electrode terminated away from the contact layer; 
     FIG. 4 is a side view of a portion of an optical modulator in accordance with an embodiment of the present invention; 
     FIG. 5 is a side view of a portion of an optical modulator in accordance with another embodiment of the present invention; 
     FIG. 6 is a side view of a portion of an optical modulator in accordance with a further embodiment of the present invention; 
     FIG. 7 is a side view of a portion of an optical modulator in accordance with a still further embodiment of the present invention; 
     FIG. 8 is a top view illustrating a Mach-Zehnder optical modulator; 
     FIG. 9 illustrates a waveguide covered with a contact layer; 
     FIG. 10 illustrates a step of removing a portion of the contact layer in accordance with the present invention; 
     FIG. 11 illustrates a step of a blanket deposition of an isolation dielectric in accordance with the present invention; 
     FIG. 12 illustrates a step of removing a portion of the isolation dielectric in accordance with the present invention; 
     FIG. 13 illustrates a step of adding an electrode stripe in accordance with the present invention; 
     FIG. 14 illustrates an alternative formation of the electrode stripe in accordance with the present invention; 
     FIG. 15 illustrates catastrophic failure of a prior art device; 
     FIG. 16 is a graph indicating experimental results confirming the improved results of the present invention; and 
     FIG. 17 is a graph indicating a statistical distribution of the experimental results of FIG.  16 . 
    
    
     These drawings are not to scale. 
     DETAILED DESCRIPTION 
     In digital optical communication, information is communicated using light pulses. To use light to transmit information, light from a source such as a laser is modified by modulating or “shuttering” the light. The modulated light source can then be transmitted along a fibre optic cable as part of a communications system. When the light signal reaches its intended destination, the information carried by the signal is recovered through the process of detection. 
     Referring to FIG. 1, according to a first embodiment of the present invention, an optical modulator device  100  known as an electro-absorption modulator is shown. The optical modulator device  100  is formed as an integrated circuit or chip having a base  104  formed of a suitable material such as indium phosphide (InP). The optical modulator  100  can be a stand alone unit or it can be monolithically integrated, for example, with another device, or, for example, with an integrated passive waveguiding section. 
     The base  104  extends into a waveguide  102  through which light either passes or is absorbed. The waveguide  102  can be in the form of a ridge as shown in FIG. 1, or alternatively, it can be buried in a cladding material. Generally, the waveguide  102  is any medium, such as a portion of a semiconductor, in which light is confined and through which light can propagate in a directed manner. The waveguide  102  consists of layers of different materials forming several regions. As illustrated in FIG. 1, an active region  106  composed of indium gallium arsenide phosphide (InGaAsP) is followed by a cladding  108  in this case indium phosphide (InP), which is in turn followed by a contact layer  110  such as indium gallium arsenide (InGaAs). The contact layer  110  acts as an electrical conductor. Above the waveguide  102  is a metallic stripe which forms an electrode  112  and is in contact with the contact layer  110 . This electrode  112  cooperates with a metal contact (not shown) beneath the base of a device  100  to permit a reverse electrical bias to be applied to the device  100 . 
     In the absence of a reverse bias, the quarternary composition of the active region  106  acts as a band gap for the light. In other words, light passes freely through the waveguide  102 . The introduction of a reverse electrical bias changes the semiconductor properties of the active region  106  so that there is direct material absorption of light in the waveguide  102 . If the reverse electrical bias is sufficiently large, the light will effectively be prevented from passing through the device  100 . For example, from a base voltage of 0V during normal operation, applying a bias of −2V can extinguish 99% of the light passing through the device  100 . 
     Referring to FIG. 2, the electrode  112  does not extend the entire length of the waveguide  102  and ends before the point of optical entry  120  of the laser light. As previously indicated, the region of electrical contact of the electrode  112  is limited to reduce capacitance and other side effects. 
     Referring to FIG. 2, the electrode  112  of the present device consists of three layers of metal: a gold (Au) layer  122 , a platinum (Pt) layer  124  and a titanium (Ti) layer  126 . Other combinations of metals are also possible depending on the specific details of the composition of the device  100 . The intermediate platinum layer  124  is necessary to prevent gold from diffusing into the device  100  through the InGaAs layer  110  which would render the device  100  inoperative. Under conventional techniques, an abrupt termination of the electrode  112  results in a tapering (not shown) of all three layers ( 122 ,  124 ,  126 ) of the electrode  112 . In particular, at the point of termination or terminal end of the electrode  112 , there is potential that the platinum layer  124  tapers away to nothing and the protection afforded by the platinum layer ( 124 ) is no longer reliable at the point of termination. Thus it is possible that gold can diffuse into the device  100  through the InGaAs layer  110  rendering the device  100  unreliable or inoperable. 
     Referring to FIG. 3, one technique for ensuring that gold does not diffuse into the device  100  is to terminate the electrode  112  in a raised fashion away from the device  100 . Since the raised terminal end  136  of the electrode  112  does not make contact with the contact layer  110 , there is no possibility of gold diffusing into the contact layer  110  from the raised end  136  of the electrode  112 . Since the protection of the platinum layer  124  there is unimportant, an abrupt termination of the electrode  112  is acceptable. 
     In addition to functioning as an electrical path, the electrode  112  also acts as a thermal path for heat within the waveguide  102 . Heat generated in the waveguide  102  flows through the contact layer  110  into the metallic electrode  112  where it is carried away. The electrode  112  is typically connected or extended to a massive bond pad (not shown) to which a wire or other electrical contact (not shown) can be made. The electrode  112  also functions to create a thermal path to the relatively massive bond pad which acts as a heat sink for the device  100 . 
     As the optical power passing through the device  100  or the voltage of the reverse bias increases, more heat is absorbed by the waveguide  102 . Referring to FIG. 15, it has been observed that the location of failure of the device  100  is away from the region of electrical contact of the electrode  112 . It has been deduced by analysis and confirmed by experimentation that this problem of local overheating can be ameliorated by providing a thermal path over the region of failure. 
     According to the present invention, the electrode  112  is extended beyond the region of desired electrical contact as illustrated in FIG.  4 . In order to avoid undesired effects of an extended electrical contact, such as increased capacitance, in a region outside the region of desired electrical contact the electrically conducting contact layer  110  is replaced by an isolation dielectric  140  such as silicon nitride (SiN) or silicon oxynitride in this region. Thus in the region of desired electrical contact the electrode  112  and contact layer  110  provides an electrical path and a thermal path whereas outside the region f desired electrical contact the electrode  112  and SiN layer  140  provide a thermal path but not an electrical path. Accordingly, heat is dissipated from the waveguide  102  beyond the region of electrical contact. In particular, the site of previous failure due to local overheating now has means for dissipating some of the heat. The device  100  in accordance with the present invention can handle greater optical power or alternatively, can operate under a greater reverse electrical bias. 
     In FIG. 4, the contact layer  110  and isolation dielectric  140  materials are shown as adjacent and abutting at the juncture or common boundary between the materials. However, due to finite mask alignment tolerances, it may be possible that a gap is formed between the two layers exposing a portion of the base layer  100 . Referring to FIG. 5, if such a gap is unacceptable, a region of overlap  150  of the contact layer  110  and isolation dielectric  140  materials can be formed during the masking process to ensure complete coverage of the base layer  108  at the juncture or common boundary  142 . 
     In FIGS. 4 and 5, the electrode  112  is spaced from the common boundary  142  in an arch which bridges the juncture. This is done to protect the electrode  112  from stress which may occur at that discontinuity. An alternative embodiment is illustrated in FIG. 6, in which the electrode  112  is suitably modified, for example by increasing its thickness or increasing its strength through annealing, and is in continuous contact with the contact layer  110  and isolation dielectric  140  materials over the common boundary  142 . This increases the contact of the electrode  112  with the waveguide  102  and prevents or reduces the possibility of a relative hot spot developing below the common boundary  142 . This also facilitates the manufacturing process as the extra effort required for construction of the arch is no longer required. 
     A further embodiment of the present invention is illustrated in FIG.  7 . According to this embodiment, as previously described, the isolation dielectric, SiN  140  extends beyond the termination of electrode  112  and the termination is not raised. This technique is particularly appropriate where the isolation dielectric  140  is not susceptible to diffusing undesirable metals. For example, since gold cannot permeate SiN, the previously required raised termination technique is no longer required. Thus, the electrode  112  can be abruptly terminated above and adjacent the SiN isolation dielectric  140 . 
     Referring to FIGS. 16 and 17, several conventional electro-absorption modulators were placed along a bar mixed with electro-absorption modulators in accordance with the present invention. This arrangement eliminated any effect from the position of the modulator along the bar. The procedure used in each case was to launch a constant amount of power and to increase the bias voltage in the modulators until the modulators failed. The light was set at 1550 nm with a launch power of 19.6 dBm and the “blowup” voltage for each of the modulators is shown in FIG.  16 . The significant improvement in performance afforded by the present invention can be seen in FIG. 17 which shows the statistical distribution of the “blowup” voltage of the conventional devices compared with devices of the present invention. 
     The previous embodiments of the present invention have been discussed in the context of an electro-absorption modulator in which there is heating of the device due to direct material absorption of light. The present invention is not, however, restricted to such a device. 
     For example, referring to FIG. 8, a Mach-Zehnder modulator  800  comprises two portions, an attenuation portion  802  and an interferometer portion  804 . Light flows into the modulator  800  in the direction indicated. The attenuator portion  802  includes a straight portion  806  of the waveguide and a semiconductor device  808 , employing technology similar to that discussed above for the electro-absorption modulator, to control the strength of the light source introduced into the rest of the device  800 . 
     The interferometer portion  804  includes a bifurcated optical path  810  to split the incoming light. Each arm is provided with a semiconductor device  812  again using known technology similar to that previously discussed, to alter the physical properties of the waveguide. In essence the index of refraction of each arm is modified so that when the separated streams of light merge into a single stream, they can be controlled to constructively or destructively interfere with each other. 
     The use of the present invention to prevent local overheating in the attenuation portion  802  of the Mach-Zehnder modulator  800  is directly analogous to the examples discussed above. It is less necessary in the interferometer portion  804 , since the interferometer uses constructive interference and not direct material absorption to modulate light, and there is relatively little heating due to absorption. Nonetheless, accidental absorption can be a problem and the present invention can be applied to decrease the possibility of local overheating in the interferometer portion  804  as well. 
     The present invention is not limited to the examples discussed above and is applicable in other devices such as lasers, photo detectors and semiconductor optical amplifiers having a waveguide. In accordance with the present invention, in a manner similar to the previous examples, an electrode of a waveguide in a photo detector or a semiconductor optical amplifier can be extended beyond the region of desired electrical contact with the waveguide while providing a corresponding non-electrically conducting portion of the contact layer to reduce local overheating in the device. 
     A further application of the present invention relates to its use to improve the functioning of lasers such as the 980 nm pump laser. As discussed above, overheating of a region of the waveguide in the vicinity of the facet is a problem. Extending the electrode of the device beyond the region of desired electrical contact, for example, to an inactive window structure while providing a corresponding non-electrically conducting portion of the contact layer would allow heat to be carried away from the overheated region without making undesired electrical contact with the device. 
     Constructing the device of the present invention requires additional steps when compared with conventional techniques. Referring to FIG. 9, the method of the present invention begins by providing a waveguide covered with a conventional contact layer  110  of InGaAs. Referring to FIG. 10, this layer  110  is removed by conventional masking and etching techniques in the region outside the region of desired electrical contact. Next, as illustrated in FIG. 11, a blanket deposition of isolation dielectric  140  is made. Next, as illustrated in FIG. 12, a further masking and etching step removes the isolation dielectric  140  from the region of desired electrical contact. An overlap  150  between the two regions can be created as illustrated in FIG. 12, or the two regions can be flush as illustrated in FIG. 4, depending on the masking procedure chosen. Finally, the electrode  112  is deposited on the covered waveguide  102  as illustrated in FIG.  13 . 
     Referring to FIG. 14, if a bridge structure is required, a suitable material, such as PMGI (poly(dimethyl glutarimide)), is used to form a temporary structure before the step of forming the electrode  112 . PMGI is a deep UV sensitive material which can be applied and then heated to a plastic state. Because of reflow during heating, it forms a rounded boss  160 . The electrode layers are then deposited over the PMGI boss  160  to form the desired arched electrode  112 . Finally, the PMGI boss  160  can be dissolved and washed away. 
     The above described embodiments of the present invention are intended to be examples only. Alterations, modifications, and variations may be effected to the particular embodiments by those of skill in the art without departing from the scope of the invention, which is defined solely in the claims appended hereto.