Patent Publication Number: US-11036006-B2

Title: Waveguide device and method of doping a waveguide device

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a national stage entry, under 35 U.S.C. § 371, of International Application Number PCT/EP2017/081215, filed on Dec. 1, 2017, which claims priority to and the benefit of U.S. Provisional Patent Application No. 62/429,703, filed Dec. 2, 2016. The entire contents of all of the applications identified in this paragraph are incorporated herein by reference. 
    
    
     FIELD 
     One or more aspects of embodiments according to the present invention relate to a waveguide device, and more particularly to a waveguide device comprising a rib waveguide active region, such active region functioning as a modulation or photodiode region, the rib waveguide active region having a first doped slab region at a first side of a ridge and a second doped slab region at a second side of the ridge; the doped slab regions located at small distances from the ridge. 
     BACKGROUND 
     The ability of silicon photonics to provide highly functional optical chips has long been recognized. Silicon photonics platforms support passive and active devices. However, the speed of active device such as modulators and photodiodes often reaches the limits of capability of the silicon active devices. There is thus a demand for faster active devices and devices that consume less power. One of the constraints on improvements in performance of photonic devices is the limit to accuracy of fabrication, particularly to the accuracy of the positioning and concentration of dopants. 
     Doped devices such as electro-absorption modulators (EAMs) and photodetectors form an important part of the development in silicon photonics and generally rely on lithographic techniques for fabrication, particularly in relation to doped regions. However, in the race to produce ever faster optical devices and components there is a need to fabricate using increasingly small dimensions. This can only be achieved if the accuracy of techniques such as area doping can be improved. As the dimensional limits of lithographic techniques are reached, there exists a need to provide improvements in the fabrication methods of optical components with smaller dimensions. 
     SUMMARY 
     According to a first aspect of the present invention, there is provided a waveguide device comprising a rib waveguide region, the rib waveguide region having: a base, and a ridge extending from the base, wherein: the base includes a first slab region at a first side of the ridge and a second slab region at a second side of the ridge; a first doped slab region that extends along the first slab region; a second doped slab region that extends along the second slab region; a first doped sidewall region that extends along a first sidewall of the ridge and along a portion of the first slab, the first doped sidewall region being in contact with the first doped slab region at a first slab interface; and a second doped sidewall region that extends along a second sidewall of the ridge and along a portion of the second slab, the second doped sidewall region being in contact with the second doped slab region at a second slab interface; and wherein the separation between the first sidewall of the ridge and the first slab interface is no more than 10 μm; and wherein the separation between the second sidewall of the ridge and the second slab interface is no more than 10 μm. In some embodiments, the separation between the first sidewall of the ridge and the first slab interface is no more than 5 μm; and the separation between the second sidewall of the ridge and the second slab interface is no more than 5 μm. The rib waveguide region may be a rib waveguide modulation region. By rib waveguide, it may be meant that an optical mode of the waveguide device is chiefly confined to the ridge. The term ‘rib’ may be used interchangeably with ‘ridge’. 
     In this way, dimensions can be achieved which are close to or smaller than the lower limits of lithographic techniques. This is highly desirable in active waveguide devices such as an electro absorption waveguide modulator (EAM), photodetector (PD), electro optical phase modulators (EOM) and electro optical switches including Mach Zehnder interferometer (MZI) switches. For example, for EAMs is established that the performance (especially the speed) of the device improves as the product of the capacitance and the resistance of the fabricated device (C j ·R j ) gets smaller. The capacitance C is reduced significantly as the sidewall doping concentration can be engineered to increase effective intrinsic region width. Series resistance is reduced since the highly doped region can be deeply doped by multiple implantations with different energies without affecting the junction capacitance. Hence the product C j ·R 5  can be reduced significantly, by at least one order of magnitude compared with previous implementations. 
     In some embodiments, the waveguide device further comprises a first electrical contact located on the first doped slab region and a second electrical contact located on the second doped region; wherein the separation distance between the first electrical contact and the first sidewall of the ridge is no more than 10 μm; and wherein the separation between the second electrical contact and the second sidewall of the ridge is no more than 10 μm. 
     In some embodiments, the separation distance between the first electrical contact and the first sidewall of the ridge is no more than 5 μm; and the separation between the second electrical contact and the second sidewall of the ridge is no more than 5 μm. 
     According to a second aspect of the present invention, there is provided a method of fabricating a waveguide device comprising: 
     providing a rib waveguide, the rib waveguide comprising: a base, and a ridge extending from the base; wherein: the base includes a first slab region at a first side of the ridge and a second slab region at a second side of the ridge; and 
     creating a first doped slab region which extends along the first slab region; the step of creating the first doped slab region comprising: 
     providing a photoresist over at least a portion of the second slab region, the photoresist extending further from the base than the ridge extends from the base; 
     implanting the first slab region with a dopant at an angle α to the first sidewall of the waveguide, using the photoresist as a mask to cast a shadow over regions not to be doped including the second sidewall of the ridge. 
     In this way, the limitations of conventional lithographic doping techniques which do not allow devices with such small components to be made, especially not in high yields, are overcome. 
     According to a third aspect of the present invention, there is provided a waveguide device comprising a rib waveguide region, the waveguide device being fabricated by the method of the second aspect. 
     Further optional features are set out below: 
     In some embodiments, the method of fabricating a waveguide device further comprises the step of: implanting a first sidewall of the ridge and a portion of the first slab region with the dopant, at an angle β to the first sidewall of the ridge, to create a first doped sidewall region which extends along a first sidewall of the ridge and along a portion of the first slab, the first doped sidewall region being in contact with the first doped slab region at a first slab interface. 
     In some embodiments, the separation between the first sidewall of the ridge and the first slab interface is no more than 10 μm. 
     In some embodiments, the separation between the first sidewall of the ridge and the first slab interface is no more than 5 μm. 
     In some embodiments, the dopant for doping the first slab region and the first sidewall region is an n-type dopant. 
     In some embodiments, the dopant of the first slab is the same material as the dopant of the first sidewall region. 
     In some embodiments, the method of fabricating the waveguide device further comprises the steps of: 
     removing the photoresist from over the second slab region; 
     creating a first doped slab region which extends along the first slab region; the step of creating the first doped slab region comprising: 
     providing a photoresist over at least a portion of the first slab region, the photoresist extending further from the base than the ridge extends from the base; 
     implanting the second slab region with a second dopant at an angle α to the second sidewall of the waveguide, using the photoresist as a mask to cast a shadow over regions not to be doped, including the first sidewall of the ridge. 
     In some embodiments, the method of doping a waveguide device further comprises the step of: implanting a second sidewall of the ridge and a portion of the second slab region with the dopant, at an angle β to the second sidewall of the ridge, to create a second doped sidewall region which extends along a second sidewall of the ridge and along a portion of the second slab, the second doped sidewall region being in contact with the second doped slab region at a second slab interface. 
     In some embodiments, the separation between the second sidewall of the ridge and the second slab interface is no more than 10 μm. Advantageously, in some embodiments, the separation between the second sidewall of the ridge and the second slab interface is no more than 5 μm. 
     In some embodiments, the dopant for doping the first slab region and the first sidewall region is an N-type dopant. The N-type dopant of the first slab doped region may be the same material as the dopant of the second sidewall region. 
     Typically, the first doped slab region has a higher concentration of dopant (N++) than the first doped sidewall (N) and the second doped slab region has a higher concentration of dopant (P++) than the second doped sidewall (P). Examples of suitable P type dopants include: boron, BF2, and phosphorus. An example of a suitable N type dopant is arsenic. The concentration ranges for the slab doped regions (typically heavily doped), for both N and P type regions, is 1e18-1e21 [1/cm 3 ]. For sidewall doping (typically lower dopant concentrations as compared to slab doping) a typical concentration range is 1e15-1e20 [1/cm 3 ]. For sidewalls, the more lightly doped, the better, to get as wide an intrinsic region as possible. This reduces the capacitance and increases the RC time constant which can be a key bandwidth driver. The aim is to increase the gain bandwidth of the device as much as possible. 
     In some embodiments, where a photoresist thickness of 5.6 μm or more is used, the implant angle α angle has a value of 17.7 degrees or greater. 
     In some embodiments, the device further comprises a regrown or epitaxial crystalline cladding layer located between the base of the waveguide device and a silicon substrate. In some embodiments, the device may further comprise a buried oxide layer, disposed on opposing horizontal sides of the epitaxial crystalline layer, and wherein the epitaxial crystalline cladding layer is formed of a material which is different from the buried oxide layer. By horizontal, a direction may be meant which is perpendicular to the direction in which the ridge extends. The epitaxial crystalline cladding layer may be formed of a material which is not buried oxide. The epitaxial crystalline cladding layer may be formed, for example, of silicon (Si) or silicon germanium (SiGe). 
     By using the method described herein, it is possible to improve device performance since series resistance is decoupled from junction capacitance. Using previous methods, the achievable series R is typically more than 60 ohm while capacitance of the junction is more than 50 fF. Using proposed method, it is possible to achieve sub 10 ohm series resistance and a junction capacitance more than 30 fF. This means factor of 10× improvement in the RC time constant. While this means device high speed behavior is not limited to RC time constant and it is transit time limited (the maximum achievable bandwidth), at the same time it also improves other aspects of device operation like higher linearity (increasing optical saturation power), improved high-speed operation at high optical powers, higher extinction ratio, ER, at higher optical power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features and advantages of the present invention will be appreciated and understood with reference to the specification, claims, and appended drawings wherein: 
         FIG. 1  depicts a schematic diagram of a waveguide device and method of fabricating the waveguide device according to the present invention; 
         FIG. 2  depicts a schematic diagram of a semiconductor wafer bearing the regions to be doped being rotated to expose the surface at the correct angel for doping. 
         FIGS. 3 a , 3 b  and 3 c    illustrate examples of the photoresist applied during the fabrication process to enable shadow doping of the waveguide device; 
         FIG. 4  shows a representative circuit for a waveguide device according to the present invention, the waveguide device taking the form of an electro absorption modulator; 
         FIG. 5  shows a representative circuit for a waveguide device of the present invention, the waveguide device taking the form of a waveguide photodiode; 
         FIG. 6  shows a schematic diagram of a variant waveguide device; and 
         FIG. 7  shows a schematic diagram of a further variant waveguide device. 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of an active waveguide device and a method of fabrication of a waveguide device provided in accordance with the present invention and is not intended to represent the only forms in which the present invention may be constructed or utilized. The description sets forth the features of the present invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features. 
     A waveguide device and method of fabricating the device is described below in relation to  FIG. 1 . Initially the basic active waveguide structure is formed as an upstanding waveguide fabricated on a silicon platform, for example a silicon on insulator (SOI) platform using standard techniques, the structure including a waveguide, the waveguide comprising: a base, and a ridge  11  extending from the base. The base includes a first slab region  12  at a first side of the ridge and a second slab region  13  at a second side of the ridge  11 . The ridge  11  includes a first sidewall and a second sidewall. 
     The basic active waveguide structure may comprise materials such as silicon germanium (in particular, for example when the waveguide device is an optical modulator) or germanium (in particular, for example, when the waveguide device is a photodiode). The techniques for fabrication of such waveguide structures are known and will not be described here in more detail. Instead, this application focusses on the novel doping structures and techniques which form part of the fabrication of the final device, whether that be a modulator, a photodiode, or another waveguide-based device. A first doped slab region  22  is created which extends along the first slab region  12 ; the step of creating the first doped slab region comprising: providing a photoresist (not shown in  FIG. 1 ) over at least a portion of the second slab region  13 , the photoresist extending further upwards (i.e. further in a direction parallel to that in which the sidewalls of the ridge extend) from the base than the ridge extends from the base and then implanting the first slab region  12  with a dopant N++ at an angle α 1  to the first sidewall of the waveguide, and thereby using the photoresist as a mask to cast a shadow over regions not to be doped including the second sidewall  21   b  of the ridge. Importantly, the lateral distance covered by the shadow on the first slab dictates where the slab doped region  22  will terminate (i.e. the position of a first slab interface). For fast devices, the goal is to get the first slab interface as close to the first sidewall as possible. For example, a separation of no more than 10 μm is desirable and a separation of no more than 5 μm is even more desirable. 
     In a separate doping step, a first sidewall of the ridge and a portion of the first slab region is implanted with a dopant at an angle β 1  to the first sidewall of the ridge, to create a first doped sidewall region  21   a  which extends along a first sidewall of the ridge and along a portion of the first slab, the first doped sidewall region  21   a  therefore contacting the first doped slab region physically and electrically at a first slab interface  42  which is laterally offset in a first direction from the first sidewall of the ridge. The fabrication process is repeated on the second side of the waveguide, starting initially with the step of removing the photoresist from over the second slab region  13  and instead providing a photoresist over at least a portion of the first slab region  22 , the photoresist extending further from the base than the ridge extends from the base. A second doped slab region  23  which extends along the first slab region can then be created using the shadow doping method by implanting the second slab region with a second dopant at an angle α 2  to the second sidewall of the waveguide, using the photoresist (not shown) as a mask to cast a shadow over regions of the second slab and second sidewall of the ridge that are not to be doped. 
     In a final doping step, the second sidewall of the ridge and a portion of the second slab region is doped by implanting them with a dopant at an angle β 2  to the second sidewall of the ridge, to create a second doped sidewall region  21   b  which extends along a second sidewall of the ridge and along a portion of the second slab, the second doped sidewall region being in contact physically and electrically with the second doped slab region  23  at a second slab interface  43  which is laterally offset in a second direction from the second sidewall of the ridge. 
     A first electrical contact  32 , typically a metal layer, is located on top of the first doped slab region  22  in electrical contact with the first doped slab region and a second electrical contact  33 , typically a metal layer, is located on top of and in electrical contact with the second doped slab region. In this way, an electrical bias applied between the two electrical contacts  32 ,  33  will provide a corresponding bias across the waveguide. For optimal working speeds of the device, it is desirable to locate the electrical contacts as close to the waveguides as possible. Again, a separation of no more than 10 μm is desirable and a separation of no more than 5 μm is even more desirable. 
     Where the electro optical waveguide device takes the form of an electro absorption modulator (EAM), the waveguide is formed of an electro-absorption material. By applying a bias across the first and second contacts, an electrical field is generated in the electro-absorption material in which the Franz Keldysh effect occurs, the presence of an electrical field thereby giving rise to an increase in the absorption of light within the electro-absorption material. 
     Where the electro optical waveguide device is a photodiode, the ridge will comprise an optically active material. Upon application of a reverse bias across the first and second contacts, an electrical field will be created between the doped regions. Absorption of light within the waveguide will cause a change in the electrical current between the contacts  32 ,  33 , the magnitude of which indicates the intensity of the light detected. 
       FIG. 2  shows examples in which a wafer can be placed relative to the angle at which the ion beam of the dopant is applied. In this way, the angle of the wafer itself during the dopant process facilitates the shadow doping procedure. In the first example (a), the wafer is angled so that its face is orthogonal (i.e. at 90 degrees or substantially 90 degrees) to the ion beam of dopant. In other words, no tilt is applied to the wafer. In the second example (b), a tilt is applied to the wafer so that the face of the wafer is at a non-orthogonal (or substantially non-orthogonal) angle to the ion beam. For example, a wafer tilt of 30 degrees may be applied, in which case, the smallest angle γ between the face of the wafer and the ion beam would be 60 degrees or substantially 60 degrees. Clearly, for the embodiment depicted in  FIG. 1 , during the deposition of the dopant for the first slab doped region and the second doped region, the smallest angle γ between the face of the wafer and the ion beam will have an angle of α1, or α2 respectively. 
     Suitable dopants could include phosphorus for N-type doping and boron for P-type doping. 
     By utilizing the methods described above, particularly the shadow masking layer or layers, it is possible to create a shadow precise enough produce a highly doped region very close to the active waveguide. If such high doping were to extend into the waveguide region, the device would not perform as required. Neither would it perform as effectively if the doping were too far from the waveguide. The ability to tilt accurately the face of the wafer at an angle γ to direction of a doping beam further facilitates the control of the angle and in combination with the shadow doping, therefore creates an improved method by which a highly doped region can be applied in close proximity to the waveguide. 
     The shadow doping mechanism is explained in more detail below with reference to  FIGS. 3 a , 3 b  and 3 c   . The embodiment described in relation to  FIGS. 3 a , 3 b  and 3 c    show one example of suitable dimensions that would result in a desirable sub-Sum measurement between the doped slab region and the adjacent sidewall of the ridge. 
       FIG. 3 a    shows an example of desirable dimensions for a particular instance of doping of the second slab  13 . In this example, a photoresist mask is applied over the first slab  12 , leaving a clearance distance of 0.3 μm between the photoresist and the first sidewall of the ridge. The photoresist has a height which is greater than that of the ridge. In the embodiment shown, a layer of cladding is applied to the top of the ridge, in this example with a thickness of 0.5 μm. The height of the photoresist is therefore chosen so that it is greater than the sum of the ridge and the cladding layer. In the embodiment shown, the ridge has a height of 2.6 μm and a thickness of 0.8 μm, and the photoresist mask  50  applied has a height of 5.6 μm. 
       FIGS. 3 b  and 3 c    depict the tolerance and design considerations that must be taken when determining the optimum angle α at which the ion dopant beam should be set in order to ensure that the opposite sidewall (in this case the first sidewall) lies entirely within the shadow of the photoresist mask but that the doped region of the slab (in this case the second slab) lies as close as possible to the second sidewall of the ridge, thereby reducing the series resistance R of the device as much as possible. 
     The minimum value for the implant angle α of  FIG. 3 a    which ensures that it does not dope the first sidewall can be calculated by the following equation:
 
tan α=maximum separation/(height of photoresist−height of ridge)  (1.1)
 
     where the height of the ridge includes the thickness of any cladding layer if a cladding layer is present. 
     For the example dimensions depicted in  FIG. 3 b   , this gives the following minimum implant angle: 
     
       
         
           
             
               
                 
                   
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     The calculation has assumed a 0.5 μm error in the separation distance between the first sidewall and the photoresist mask. For the calculation of the smallest possible angle α, the maximum possible separation for this example is used (i.e. 0.3 μm+0.5 μm=0.8 μm). 
       FIG. 3 c    illustrates the other extreme within the tolerance of the system (i.e. that the photoresist ends up deposited 5 μm in the other direction). In this case, there is no separation between the photoresist and the first sidewall. Some of the photoresist is deposited on top of the ridge. Using equation 1.1 above, it is therefore possible to calculate the worst case scenario in terms of the minimum separation between the doped (second) slab and the second sidewall that can be achieved when doping at the minimum implant angle. Using the dimensions of  FIG. 3 c   , and the angle calculated in 1.3 above, this gives: 
     
       
         
           
             
               
                 
                   
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     Note that it is assumed that the resist which is located on top of the cladding on the ridge will have a thickness corresponding to the height of the photoresist on the first slab (in this case 5.6 μm).
 
 x= 2.78 um  (1.5)
 
     So, for the dimensions shown in  FIG. 3 c   , an implantation angle of 17.7 degrees will lead to a worst case scenario of 2.78 μm for the separation between the second sidewall and the second doped slab region. 
     Doping is achieved in a standard implanter with capability to tilt and rotate the substrate holder. For a given dopant the “dose” of dopant received by the regions of semiconductor to be doped depends upon the energy of the ion beam and the time of exposure.
 
The slab doped regions (typically heavily doped) and sidewall doped regions (typically lightly doped) may use different dopants. That is to say, it would be possible to have 2 different P dopants and 2 different N dopants. One example of a benefit provided by different dopants could be the ability to more easily obtain different depths of penetration that may be required for performance optimization.
 
       FIG. 4  shows the representative circuit of a waveguide EA modulator where the EAM modulator shown is a top view and the fabrication of the device corresponds to that of the waveguide device  1  shown in  FIG. 1 . A bias V j  is applied across the contacts  32 ,  33 , giving rise to a resulting capacitance and Resistance R s . 
     It is established that the performance (especially speed) of the device improves as C j ·R 5  gets smaller. The capacitance C is reduced significantly as the sidewall doping concentration can be engineered to increase effective intrinsic region width. Series resistance is reduced since the highly doped region can be deeply doped by multiple implantations with different energies without affecting the junction capacitance. Hence the product C j ·R 5  can be reduced significantly. 
     The embodiment of  FIG. 4  shows an example of a large signal lumped circuit model of an EAM. 
       FIG. 5  shows the representative circuit of a waveguide photodiode modulator where the fabrication of the photodiode will again correspond to that of the waveguide device  1  shown in  FIG. 1 . In this case, a current I PD  is generated by putting optical power inside photodiode waveguide for a given reverse bias Vj across the contacts  32 ,  33 . As with the modulator example, the device, in use, will therefore have an inherent capacitance Cj and Resistance R s . Again, the performance of the device improves as C j ·R s gets smaller. 
     As will be appreciated, the method of fabrication described above can be used to fabricate many variants of electro-absorption modulator or photodetector. For example: proud waveguide, single silicon sidewall, and ‘BOX-less’ devices i.e. those with a epitaxial crystalline layer.  FIGS. 6 and 7  show examples of these devices made using the same method of fabrication described above and can generally be referred to as proud waveguide devices. In the devices shown there is no buried oxide below the ridge of the waveguide, instead there may be a regrown or epitaxial crystalline cladding layer. The slab and a portion of the ridge is formed of a first material M 1 , and a region beneath the first slab region and the second slab region is formed of a second material M 2 . The second material M 2  may be buried oxide (BOX) e.g. silicon oxide. The remaining part of the ridge not formed of material M 1  may be formed of a different material e.g. Si or SiGe. Of course, in other examples, buried oxide may be present below the ridge such that the second material M 2  forms a substantially continuous layer. In  FIG. 6 , at least one sidewall  601  of the ridge is formed of doped silicon. Generally the sidewall doping concentration is smaller than the slab doping. This can be achieved by separating the implantation processes. The structures shown are similar to those disclosed in U.S. 62/429,701, the entire contents of which is incorporated herein by reference. 
     Although exemplary embodiments of an active waveguide device have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that an active waveguide device constructed according to principles of this invention may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.