Patent Publication Number: US-2023163563-A1

Title: Optical Semiconductor Chip

Description:
BACKGROUND ART 
     The present invention relates to an optical semiconductor chip used in an optical transmitter or the like. The capacity of optical communication is increasing from backbone networks to access system networks. Larger capacities and higher performance are also required for optical transmitters, and a high-speed optical modulator is required as one key technology. Direct modulation is a method of modulating an optical signal from a light source (semiconductor laser) itself. For example, a method of directly modulating an output intensity of a semiconductor laser by controlling a current of the laser is known. 
       FIG.  1    shows diagrams of a configuration of a direct modulation laser chip in the conventional art.  FIG.  1 ( a )  shows a top view of a part of an optical semiconductor chip  10  in which an optical semiconductor laser is formed, and  FIG.  1 ( b )  shows a cross-sectional view of the substrate taken along the line Ib-Ib. First, referring to the cross-sectional view of  FIG.  1 ( b ) , an optical semiconductor laser has an optical waveguide structure in which an active layer  15  and a p-type semiconductor layer  14  are formed on an n-type semiconductor substrate  16 . Both sides of an optical waveguide are embedded with an insulation layer  13 . It should be noted that, the above optical waveguide constitutes a semiconductor laser, but  FIG.  1    shows only a modulation unit which is a part of the entire optical waveguide ( FIG.  2   ) constituting the laser. Oscillation light generated in the laser unit is modulated by applying a baseband signal (hereinafter referred to as a modulation signal) to a modulation electrode  12  formed on the p-type semiconductor layer  14 . Referring to  FIG.  1 ( a ) , in order to input a modulation signal via wire bonding, bumps or the like, the electrode is extended from the modulation electrode  12  substantially perpendicular to the optical waveguide, and an electrode pad  11  is formed. The electrode pad  11  has a size of about 40 to 100 μm square, and has a structure directly connected to the modulation electrode  12 . 
     A portion under the electrode pad  11  is embedded with a material  17  having a lower dielectric constant than that of the semiconductor insulation layer  13 , and thereby the parasitic capacitance of the electrode pad  11  is reduced. When the parasitic capacitance of the electrode pad  11  receiving a modulation signal is reduced, deterioration of frequency response characteristics is prevented in a high frequency range exceeding 10 GHz. 
     The above direct modulation laser chip is formed on an optical semiconductor substrate having a function of conversion between light and electricity, and although not limited, in many cases, a chip is realized by disconnecting a plurality of circuits formed on a wafer-shaped optical semiconductor substrate. The direct modulation laser chip shown in  FIG.  1    is also called an optical semiconductor chip. 
       FIG.  2    shows diagrams of a configuration of a subassembly in which an optical semiconductor chip including a direct modulation laser is mounted.  FIG.  2 ( a )  is a top view of a subassembly  20 , and  FIG.  2  ( b )  shows a cross-sectional view of the subassembly  20  taken along the line IIb-IIb. Referring to  FIG.  2 ( b ) , in the subassembly  20 , a radio frequency (RF) wiring board  22  and the optical semiconductor chip  10  are mounted on the subcarrier  21 . The RF wiring board  22  and the optical semiconductor chip  10  are connected by a terminator integrated chip  23  via gold bumps  24   a  and  24   b.    
     Referring to  FIG.  2 ( a ) , the RF wiring board  22  includes an RF pattern  25  receiving a modulation signal, and similarly, an RF pattern is formed in the terminator integrated chip  23 . In the terminator integrated chip  23 , a resistor  26  for terminating a modulation signal is formed. The RF pattern  25  is electrically connected to the electrode pad  11  of the optical semiconductor chip  10  via the gold bump  24   a , the terminator integrated chip  23 , and the gold bump  24   b.    
       FIG.  3    is a diagram showing an equivalent circuit model of a subassembly in which an optical semiconductor chip in the conventional art is mounted. An equivalent circuit model  30  corresponds to the entire subassembly  20  in  FIG.  2   , and is divided into a laser chip part equivalent circuit  34  corresponding to the optical semiconductor chip  10  in  FIG.  1    and another equivalent circuit part corresponding to the RF wiring board  22  and the terminator integrated chip  23 . The other equivalent circuit part is composed of the RF pattern  25 , an impedance  32  corresponding to the terminator integrated chip  23 , and an inductor component  33  of a gold bump. 
     CITATION LIST 
     Non Patent Literature 
     
         
         [NPL 1] W. Kobayashi et. al., “High-speed operation at 50 Gb/s and 60-km SMF transmission with 1.3-μm InGaAlAs-based DML”, 2012. in Proc. ISLC2012 TuB1 
         [NPL 2] S. Kanazawa et. al., “Equalizer-free transmission of 100-Gbit/s 4-PAM signal generated by flip-chip interconnection EADFB laser module”, Feb. 15, 2017, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 35, NO. 4 
       
    
     SUMMARY OF THE INVENTION 
     Technical Problem 
     Here, focusing on an equivalent circuit  34  of an optical semiconductor chip  10 , a parasitic capacitance  36  corresponding to an electrode pad  11  has already been considerably reduced by the configuration such as embedding a low-dielectric-constant material  17  shown in  FIG.  1   . For bandwidth characteristics of an optical modulator, a depletion layer capacitance  37  in an equivalent circuit  35  corresponding to a laser unit having an active layer  15  has a dominant influence in the equivalent circuit  34  of the laser unit in the optical semiconductor chip  10 . 
     When a direct modulation laser having a higher speed and a wider bandwidth is realized, there is a new problem that frequency response characteristics are limited by the depletion layer capacitance in the optical waveguide of the laser unit. The present invention has been made in view of such problems, and an object of the present invention is to realize an optical modulator using a direct modulation laser having a higher speed and a wider bandwidth. 
     Means for Solving the Problem 
     In order to achieve such an object, one embodiment of the present invention provides an optical semiconductor chip including: a laser light source having an optical waveguide structure, the structure having a first type semiconductor base layer, an active layer and a second type semiconductor layer arranged in this order; an electrode pad receiving a modulation signal; a modulation electrode being formed on the second type semiconductor layer; and a high frequency line connecting the electrode pad and the modulation electrode and providing inductance in series with respect to a depletion layer capacitance of the optical waveguide. The optical semiconductor chip can operate as a direct modulation laser. 
     Effects of the Invention 
     As described above, the present invention realizes an optical modulator having a higher speed and a wider bandwidth. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows diagrams of a configuration of a direct modulation laser chip in the conventional art. 
         FIG.  2    shows diagrams of a configuration of a subassembly in which a direct modulation laser chip is mounted. 
         FIG.  3    is a diagram showing an equivalent circuit model of a subassembly in the conventional art. 
         FIG.  4    shows diagrams of a configuration of an optical semiconductor chip including a direct modulation laser of the present disclosure. 
         FIG.  5    is a diagram showing an equivalent circuit model of a direct modulation laser subassembly of the present disclosure. 
         FIG.  6    is a diagram showing frequency response of the direct modulation laser subassembly of the present disclosure. 
         FIG.  7    shows diagrams of a configuration of an optical semiconductor chip including a direct modulation laser of Example 2. 
         FIG.  8    shows diagrams of frequency response of a direct modulation laser subassembly of Example 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An optical semiconductor chip including a direct modulation laser of the present disclosure includes a high frequency line between an electrode pad receiving a modulation signal and a modulation electrode on an optical waveguide having an active layer. A depletion layer capacitance of the optical waveguide is cancelled by an inductor component of the high frequency line. When a portion directly below the high frequency line is embedded with a low-dielectric-constant material or is made hollow, the parasitic capacitance is further reduced. The high frequency line may have a zigzag (meander) shape as well as a linear shape. The electrode pad on the optical semiconductor chip can be connected to other substrates including RF lines via bumps or wire bonding. 
       FIG.  4    shows diagrams of a configuration of the optical semiconductor chip including the direct modulation laser of the present disclosure. Similar to  FIG.  1    shown in the configuration in the conventional art,  FIG.  4 ( a )  shows a top view of a part of an optical semiconductor chip  40  in which a direct modulation laser is formed, and  FIG.  4 ( b )  shows a cross-sectional view of the substrate taken along the line IVb-IVb. First, referring to the cross-sectional view of  FIG.  4 ( b ) , the direct modulation laser has an optical waveguide structure in which an optical active layer  45  and a p-type semiconductor layer  44  are formed on an n-type semiconductor substrate  46 . One side of the optical waveguide is embedded with an insulation layer  43 . On the other side, a portion directly below a region from the electrode pad to just before a modulation electrode  42  is replaced with a low-dielectric-constant material  47  and a hollow portion  49 . Like the direct modulation laser in the conventional art shown in  FIG.  1   , the above optical waveguide structure constitutes a semiconductor laser. Oscillation light in the laser unit is modulated by a modulation signal applied to the modulation electrode  42  formed on the p-type semiconductor layer  44 . The difference from the configuration of the optical semiconductor chip in the conventional art is a configuration between an electrode pad  41  and the modulation electrode  42 . 
     Referring to the top view of  FIG.  4 ( a ) , a high frequency line  48  is provided between the electrode pad  41  receiving a modulation signal via wire bonding, bumps or the like, and the modulation electrode  42 . The high frequency line  48  connects the electrode pad  41  and the modulation electrode  42  in a bridge shape by forming a portion directly therebelow as the hollow portion  49 . The high frequency line  48  operates as a high-frequency-transmission line having an inductive impedance up to a predetermined length at a frequency corresponding to a modulation symbol rate. That is, the high frequency line  48  operates as an inductor (inductive reactance) added in series with respect to the laser unit when viewed from the modulation input side. 
     A portion directly below the electrode pad  41  is embedded with the low-dielectric-constant material  47 , and a portion directly below the high frequency line  48  is the hollow portion  49 . According to the configuration directly below the electrode from the electrode pad  41  to the modulation electrode  42 , the capacitive impedance component in the electrode pad  41  and the high frequency line  48  can be made as small as possible. After an electrode on the entire top part of the chip is formed, by selectively etching and removing only a portion under the high frequency line  48  and leaving the high frequency line  48  in a bridge shape, such a hollow portion  49  can be formed. In addition, as shown in  FIG.  4   , instead of making the entire portion directly below the high frequency line  48  hollow, only a portion directly below the high frequency line  48  may be hollow, or the low-dielectric-constant material  47  may be embedded up to a location right next to the modulation electrode  42  without the hollow portion. When the low-dielectric-constant material  47  or the hollow portion  49  is combined with the high frequency line  48  that adds inductance to the laser unit in series, the capacitive impedance component of the electrode pad  41  and the high frequency line  48  can be further reduced. 
     Therefore, the optical semiconductor chip of the present disclosure is an optical semiconductor chip including a laser light source having an optical waveguide structure, the structure having a first type semiconductor base layer, an active layer and a second type semiconductor layer arranged in this order, an electrode pad receiving a modulation signal, a modulation electrode being formed on the second type semiconductor layer, and a high frequency line connecting the electrode pad and the modulation electrode and providing inductance in series with respect to a depletion layer capacitance of the optical waveguide can be implemented. Here, the first type and the second type correspond to the type of doping of each part of the above optical waveguide. In the example of  FIG.  4   , the first type may be an n-type semiconductor, and the second type may be a p-type semiconductor. These types can be reversed. The semiconductor base layer corresponds to a semiconductor substrate on which a direct modulation laser chip is formed. 
       FIG.  5    is a diagram showing an equivalent circuit model of a subassembly including the optical semiconductor chip of the present disclosure. In the optical semiconductor chip  40  of the present disclosure shown in  FIG.  4   , the configuration between the electrode pad  41  and the modulation electrode  42  is different from that of a direct modulation laser chip  10  in the conventional art. Therefore, the optical semiconductor chip  40  can be mounted in the same configuration as the subassembly shown in  FIG.  2   . Therefore, an optical semiconductor chip  10  in  FIG.  2    is replaced with the optical semiconductor chip  40  with the configuration in  FIG.  4   , and can be mounted as a subassembly as in  FIG.  2    together with an RF wiring board  22  on a subcarrier  21  and a terminator integrated chip  23 . 
     In an equivalent circuit  50  shown in  FIG.  5   , an impedance  52  corresponding to an RF pattern  25  and a terminating resistor  26 , and an inductor component  53  of a gold bump are the same as those of the equivalent circuit in  FIG.  3   . Only an equivalent circuit  54  of a chip part corresponding to the optical semiconductor chip  40  including a direct modulation laser is different from the equivalent circuit of the subassembly in the conventional art in  FIG.  3   . That is, in the equivalent circuit  54  of the laser chip part corresponding to the optical semiconductor chip  40 , an inductance  58  corresponding to the high frequency line  48  is included in series between a parasitic capacitance  56  of the electrode pad  41  and an equivalent circuit  55  of the laser unit corresponding to the optical waveguide. A depletion layer capacitance  57  in the equivalent circuit  55  of the laser unit and the inductance  58  are connected in series. Therefore, the decrease in impedance of a capacitive impedance  57  in a high frequency range due to the depletion layer capacitance can be cancelled (resonated) by the increase in impedance of the inductance  58  of the high frequency line  48 . Thereby, the reactance component when the chip is viewed from the modulation input side can be made substantially constant, and it is possible to improve frequency response characteristics of modulation characteristics. Hereinafter, examples will be described in more detail. 
     Example 1 
     Referring to  FIG.  4    again, as an example, in the optical semiconductor chip  40  including the direct modulation laser of the present disclosure, InP, GaAs or Si can be used for a substrate. The length of the direct modulation laser (the modulation electrode  42 ) was 100 μm, the signal line width of the high frequency line  48  was 10 μm, the line length was 500 μm, the thickness of the low-dielectric-constant material  47  was 3 μm, and the specific dielectric constant was 2.3. The electrode pad  41  had a circular shape with a diameter of 60 μm. As the example of the low-dielectric-constant material  47 , an organic material including a polyimide can be used. 
     In order to compare frequency response characteristics of the optical modulator unit of the optical semiconductor chip  40  of the present disclosure shown in  FIG.  4    with those in the conventional art, the optical semiconductor chip  40  was mounted in the form of the subassembly shown in  FIG.  2   , and the performance of the direct modulation laser was checked. For comparison, a subassembly using the direct modulation laser chip  10  shown in  FIG.  1    having the same structure as the optical semiconductor chip  40  except that the high frequency line  48  was not provided was formed. In a subassembly  20  shown in  FIG.  2   , for the RF wiring board  22  and the terminator integrated chip  23 , a ceramic material such as aluminum nitride or a quartz plate can be used. For the subcarrier  21 , an aluminum nitride substrate can be used. Here, the term “subassembly” refers to an intermediate functional block form for realizing an optical transmitter, an optical transmission device and the like including a direct modulation laser. The subassembly can be simply referred to as an assembly, a module or the like. 
     The optical waveguide of the direct modulation laser unit of the optical semiconductor chip  40  had a configuration in which the active layer  45  composed of a GaInAsP quantum well structure and the p-type semiconductor layer  44  composed of p-type InP were sequentially formed on the n-type InP substrate  46 . The configuration of each part of these optical waveguides is an example, and in order to produce an optical waveguide of the direct modulation laser as described above, various configurations can be formed using InP, GaAs or Si for a substrate. In addition, in the configuration of the optical semiconductor chip  40  in  FIG.  4   , the substrate side was an n-type semiconductor, but the substrate side can also be a p-type semiconductor. In the electrode pad  41 , both a DC bias current (10 to 100 mA) and a modulation signal (1.0 to 3.5 Vpp) were superimposed and applied to the modulation electrode  42  via the RF wiring board  22  from the outside. 
       FIG.  6    is a diagram showing frequency response characteristics of a subassembly including the optical semiconductor chip including the direct modulation laser of the present disclosure.  FIG.  6    shows frequency characteristics of electrical/optical response (E/O response) in dB. The E/O response can be acquired by an optical component analyzer or the like. In  FIG.  6 ( a ) , the 3 dB bandwidth of the subassembly using the direct modulation laser chip in the conventional art was 32 GHz. On the other hand, when the optical semiconductor chip of the present disclosure in  FIG.  4    was used, the bandwidth was 33 GHz, and the 3 dB bandwidth was widened and improved by about 1 GHz. In  FIG.  6   , it was confirmed that, according to the configuration including a high frequency line between the electrode pad receiving a modulation signal and the modulation electrode on the optical waveguide of the direct modulation laser, the modulation frequency response of the direct modulation laser was improved. In the next example of the optical semiconductor chip, a configuration example in which a meander structure was used as a high frequency line will be shown. 
     Example 2 
       FIG.  7    shows diagrams of a configuration of an optical semiconductor chip including a direct modulation laser of Example 2.  FIG.  7 ( a )  is a top view of a part of the optical semiconductor chip  70  in which a direct modulation laser is formed, and  FIG.  7 ( b )  shows a cross-sectional view of the substrate taken along the line VIIb-VIIb. Referring to the cross-sectional view of  FIG.  7 ( b ) , the optical semiconductor chip  70  had an optical waveguide structure in which an active layer  75  and a p-type semiconductor layer  74  were formed on an n-type semiconductor substrate  76 . Both sides of the optical waveguide were embedded by an insulation layer  73 . In this configuration, an electrode pad  71  and a meander wiring  77  were set to be higher than a modulation electrode  72 . The above optical waveguide constituted a semiconductor laser. Oscillation light generated in the laser unit was modulated by applying a modulation signal to the modulation electrode  72  formed on the p-type semiconductor layer  74 . The above optical waveguide structure of the optical semiconductor chip  70  was the same as that of the structure of the optical semiconductor chip  40  of Example 1 in  FIG.  4    except for the configuration of the insulation layer on both sides. The major difference from the optical semiconductor chip  40  in FIG.  4  was a configuration between the electrode pad  71  and the modulation electrode  72 . 
     In this example, the electrode pad  71  and the modulation electrode  72  were connected by the meander wiring  77 . In the semiconductor chip  70  in  FIG.  7   , the length of the direct modulation laser, that is, the modulation electrode  72 , was 75 μm, and the high frequency line was formed as the meander wiring  77  having the three types of different inductances 0.12, 0.18, and 0.3 nH. The thickness of the electrode pad  71  and the insulation layer  73  under the meander wiring  77  was set to 10 μm, which was higher than the top surface of the optical waveguide constituting the optical modulator, and the parasitic capacitance generated from the electrode pad  71  or the like was reduced. As in Example 1, in this configuration, the RF wiring board  22  and the terminator integrated chip  23  were connected using flip chip mounting, and a modulation signal was input to the electrode pad  71 . 
       FIG.  8    shows diagrams of frequency response characteristics of a subassembly including the optical semiconductor chip including the direct modulation laser of Example 2.  FIGS.  10 ( a ) and  10 ( b )  show frequency characteristics of each E/O response in different frequency ranges in dB. The meander wiring  77  having the three types of inductances 0.12, 0.18, and 0.3 nH is shown. In addition, characteristics of the subassembly when the direct modulation laser chip  10  in the conventional art in  FIG.  1    was mounted in the form shown in  FIG.  2    are shown. In  FIG.  8 ( b ) , the 3 dB bandwidth of the subassembly using the direct modulation laser chip in the conventional art was 40.2 GHz. On the other hand, when the optical semiconductor chip including the meander wiring  77  of Example 2 shown in  FIG.  6    was used, the bandwidth was 40.7 GHz, and the bandwidth was improved by about 0.5 GHz. 
     An optical semiconductor chip in which the inductance of the meander wiring  77  was set to 0.18 nH and 0.3 nH was mounted on the same subassembly, and frequency response characteristics were measured. In the results, in the case of 0.18 nH, the 3 dB bandwidth was the same as in the case of the configuration in the conventional art, and on the other hand, in the case of 0.3 nH, the 3 dB bandwidth was narrower than in the configuration in the conventional art. Therefore, it can be understood that, in the direct optical modulator composed of an InP substrate, the inductance added by the high frequency line was effectively 0.18 nH or less, and maximum improvement in the 3 dB bandwidth characteristics around 0.12 nH was expected. It was confirmed from the measured value that the inductance component added by the high frequency line was selected according to the depletion layer capacitance corresponding to the configuration of the optical modulator, and the modulation bandwidth of the direct modulation laser can be maximized. 
     Here, it has been confirmed that the amount of improvement in the 3 dB bandwidth when an inductance component is added in series depends on the length of the laser unit (modulation electrode). The results of  FIG.  8    show an example when the laser length was 100 μm. For example, in the case of a shorter laser length, the above optimal inductance value around 0.12 nH changed to a smaller value. The configuration from the electrode pad to the modulation electrode in the direct modulation laser chip in the conventional art was for reducing unnecessary inductance and the capacitance as much as possible, and securing an electrode structure sufficient for wire bonding. Therefore, the length between the electrode pad and the modulation electrode was short as possible and reduced to a maximum of 30 μm or less. It should be noted that the direct modulation laser chip of the present disclosure differed from that in the conventional art in that the high frequency line operated as an inductor (inductive reactance) added in series to the laser unit when viewed from the modulation input side. 
     As described above, when the inductor component of the high frequency line was provided in series between the electrode pad in the direct modulation laser chip and the modulation electrode on the optical waveguide that functions as the optical modulator, the depletion layer capacitance of the optical waveguide was cancelled. The line width of the inductor component is not limited as long as it has a line width that provides a predetermined characteristic impedance at, for example, a frequency of the modulation signal (for example, the symbol frequency). In addition, the line length may be in a range in which the impedance of the high frequency line becomes an inductive reactance. 
     In order to reduce the parasitic capacitance in the optical waveguide and in order to add inductance in series, the width of the high frequency lines  48  and  77  is preferably narrow, but there is a problem of the physical strength of the electrode in a line width of microns or less. Therefore, the width of the high-frequency-line units  48  and  77  is preferably in a range of the diameter of the electrode pad  71  or less and 1 μm or more. 
     As described above in detail, according to the configuration of the optical semiconductor chip of the present disclosure, it is possible to realize a direct modulation laser having a higher speed and a wider bandwidth. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be generally used in an optical communication system. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  40 ,  70  Optical semiconductor chip 
           11 ,  41 ,  71  Electrode pad 
           12 ,  42 ,  72  Modulation electrode 
           13 ,  43 ,  73  Insulation layer 
           14 ,  44 ,  74  p-type semiconductor layer 
           15 ,  45 ,  75  Active layer 
           16 ,  46 ,  76  n-type semiconductor substrate 
           17 ,  47  Low-dielectric-constant material 
           20  Subassembly 
           21  Subcarrier 
           22  RF wiring board 
           23  Terminator integrated chip 
           36 ,  56  Parasitic capacitance 
           37 ,  57  Depletion layer capacitance 
           48 ,  77  High frequency line 
           49  Hollow portion