Patent Publication Number: US-2022229317-A1

Title: Optical Inspection Circuit and Optical Inspection Method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a national phase entry of PCT Application No. PCT/JP2019/020473, filed on May 23, 2019, which application is hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an optical inspection circuit and an optical inspection method to inspect an optical circuit. 
     BACKGROUND 
     A silicon photonics technology is anticipated as a basic technology for cost reduction of optical devices in the future. However, in order to genuinely realize the cost reduction, reduction in assembly and inspection costs of optical modules is demanded. In particular, in order to reduce the inspection costs, wafer level inspection which allows a multitude of chips to be measured in a short period of time is required. 
     Although there have been some suggestions regarding a wafer level inspection technology for the optical devices (Non-Patent Literatures 1, 2, and 3), there have been few technologies which can be applied to active devices such as optical modulators. This is because each of the optical modulators is a device which converts an electrical signal to an optical signal and a response speed demanded in general is a high speed of 10 gigahertz or more. 
     CITATION LIST 
     Non-Patent Literature 
     
         
         Non-Patent Literature: 1: Y. Maeda et al., “Novel fiber alignment method for on-wafer testing of silicon photonic devices with PN junction embedded grating couplers”, 2018 IEEE 15th International Conference on Group IV Photonics, 18162636, pp. 81-82, 2018. 
         Non-Patent Literature 2: T. Miura et al., “Novel quick and precise method for evaluating optical characteristics”, 2018 IEEE 15th International Conference on Group IV Photonics, 18162651, pp. 95-96, 2018. 
         Non-Patent Literature 3: H. Fukuda et al., “Estimation of Optical Modulator Efficiency From Electrical Characteristics”, 2018 IEEE 15th International Conference on Group IV Photonics, 18162656, pp. 11-12, 2018. 
       
    
     SUMMARY 
     Technical Problem 
     As described above, since in wafer level inspection for active devices such as optical modulators, the demanded response speed is a high speed of 10 gigahertz or more, a line which transmits the high-speed electrical signal without loss is desired. In order to apply the line, which transmits the high-speed electrical signal without the loss, to the wafer level inspection, a probe card which is excellent in high frequency characteristics is needed. However, this kind of the probe card and a high frequency cable which connects the probe card and a device targeted for the inspection are expensive. In addition thereto, in order to conduct inspection whose reproducibility is high, extreme caution in arranging the probe card and the high frequency cable is desired, the inspection cannot be easily conducted, and such inspection is hardly accepted in a mass production process. 
     In order to solve the above-described problems, the present invention was devised, and an object of embodiments of the present invention is to enable wafer level inspection for optical circuits such as optical modulators to be further easily and further inexpensively implemented. 
     Means for Solving the Problem 
     An optical inspection circuit according to an embodiment of the present invention includes: an optical modulator comprising an optical waveguide formed on a substrate, the optical waveguide having a core comprising a semiconductor; a first optical waveguide constituted of an optical waveguide having a core comprising the semiconductor and is optically connected to the optical modulator; a second optical waveguide constituted of an optical waveguide having a core comprising the semiconductor and is optically connected to the optical modulator, a photodiode formed on the substrate in a vicinity of the optical modulator, a wire electrically connecting the optical modulator and the photodiode; and a third optical waveguide constituted of an optical waveguide having a core comprising the semiconductor and is optically connected to the photodiode. 
     In one configuration example of the above-mentioned optical inspection circuit, a plurality of the optical modulators is formed, and the wire connects each of the plurality of optical modulators and the photodiode. 
     In one configuration example of the above-mentioned optical inspection circuit, a plurality of the photodiodes is formed, and the wire connects the optical modulator and each of the plurality of photodiodes. 
     In one configuration example of the above-mentioned optical inspection circuit, an optical distributor which distributes inputted signal light to the first optical waveguide and the third optical waveguide is further included. 
     In one configuration example of the above-mentioned optical inspection circuit, the semiconductor is formed of silicon, and the photodiode is a germanium photodiode. 
     An optical inspection method according to an embodiment of the present invention includes: a first step of making continuous light incident to an optical modulator comprising an optical waveguide formed on a substrate, the optical waveguide having a core comprising a semiconductor; a second step of making modulated signal light incident to a photodiode formed on the substrate in a vicinity of the optical modulator and is electrically connected to the optical modulator, and a third step of evaluating modulated light outputted from the optical modulator. 
     Effects of the Invention 
     As described above, according to embodiments of the present invention, since on the substrate on which the optical modulator is formed, the photodiode is formed, and the optical modulator and the photodiode are electrically connected by the wire, inspection of an optical circuit such as an optical modulator at a wafer level can be further easily and further inexpensively implemented. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a configuration diagram illustrating a configuration of an optical inspection circuit according to an embodiment of the present invention. 
         FIG. 2  is a flowchart for explaining an optical inspection method according to the embodiment of the present invention. 
         FIG. 3  is a characteristic diagram showing a result of measuring a voltage of an electrical signal, which a photodiode outputs, with respect to power of light inputted to the photodiode. 
         FIG. 4  is a characteristic diagram showing a result of measuring relationship between a voltage of an electrical signal which is inputted (applied) to an optical modulator and light which is modulated by the optical modulator and is outputted. 
         FIG. 5  is a characteristic diagram showing a light output waveform of modulated light obtained when a light pulse of one mW at peak power is inputted to the photodiode connected to the optical modulator by a wire, the modulated light, as a result of this, being outputted from the optical modulator. 
         FIG. 6  is a configuration diagram illustrating a configuration of another optical inspection circuit according to the embodiment of the present invention. 
         FIG. 7  is a configuration diagram illustrating a configuration of still another optical inspection circuit according to the embodiment of the present invention. 
         FIG. 8  is a configuration diagram illustrating a configuration of yet another optical inspection circuit according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an optical inspection circuit according to an embodiment of the present invention will be described with reference to  FIG. 1 . This optical inspection circuit includes an optical modulator  102  formed on a substrate  101  and a photodiode  103  formed on the substrate  101  in the vicinity of the optical modulator  102 . 
     The optical modulator  102  comprises an optical waveguide, and the optical waveguide has a core comprising a semiconductor. The optical modulator  102  can be configured of, for example, a Mach-Zehnder optical modulator in which a carrier abstraction type phase shifter including a pn junction formed in a rib type optical waveguide whose core is formed of silicon and a multi-mode interferometer are combined. This optical modulator can be manufactured by the heretofore known semiconductor device manufacturing technology such as the widely known lithography technology, ion implantation technology, thin film deposition technology, crystal growth technology, and etching technology. 
     The photodiode  103  is, for example, a germanium photodiode which is configured of a germanium layer selectively formed on the core comprising a semiconductor (for example, silicon) and a pn junction formed on both sides and upper and lower sides of this layer. The photodiode  103  can be manufactured by the heretofore known semiconductor device manufacturing technology such as the widely known lithography technology, ion implantation technology, thin film deposition technology, crystal growth technology, and etching technology. 
     In addition, optically connected to the optical modulator  102  is a first optical waveguide  104  as an input waveguide, which is constituted of an optical waveguide having a core comprising a semiconductor. In addition, optically connected to the optical modulator  102  is a second optical waveguide  105  as an output waveguide, which is constituted of an optical waveguide having a core comprising a semiconductor. In addition, optically connected to the photodiode  103  is a third optical waveguide  106  as an input waveguide, which is constituted of an optical waveguide having a core comprising a semiconductor. 
     In addition, the optical modulator  102  and the photodiode  103  are electrically connected by a wire  107 . An electrical signal which is photoelectrically converted by the photodiode  103  and is outputted is electrically transmitted by the wire  107  and is inputted to the optical modulator  102 . The optical modulator  102  modulates continuous light, inputted via the first optical waveguide  104 , by the electrical signal inputted via the wire  107  and outputs the modulated continuous light to the second optical waveguide  105 . 
     For example, the wire  107  is a high frequency line comprising a signal wire  171  and grounding wires  172  and  173 . The signal wire  171  connects an electrode pad  121  of the optical modulator  102  and an electrode pad  131  of the photodiode  103 . The grounding wire  172  connects an electrode pad  122  of the optical modulator  102  and an electrode pad  132  of the photodiode  103 . The grounding wire  173  connects an electrode pad  123  of the optical modulator  102  and an electrode pad  133  of the photodiode  103 . 
     In addition, optically connected to the third optical waveguide  106  via an optical fiber  109  is an optical modulator  110 . The optical modulator  110  modulates continuous light emitted from a light source  111  and outputs the modulated continuous light to the optical fiber  109 . The light source  111  comprises, for example, a semiconductor laser. 
     In addition, optically connected to the first optical waveguide  104  is an optical fiber  112 , and inputted to the optical fiber  112  is continuous light emitted from a light source  113 . The light source  113  comprises, for example, a semiconductor laser. 
     In this optical inspection circuit, first, the modulated light (signal light) emitted from the light source  111  and modulated by the optical modulator  110  is received via the optical fiber  109  and the third optical waveguide  106  by the photodiode  103 . The modulated light received by the photodiode  103  is photoelectrically converted to a modulated electrical signal and is outputted via the wire  107  to the optical modulator  102 . 
     In addition, the continuous light emitted from the light source  113  is inputted via the optical fiber  112  and the first optical waveguide  104  to the optical modulator  102 . The continuous light inputted to the optical modulator  102  is modulated by the optical modulator  102 , which is driven by the inputted modulated electrical signal, is outputted from the second optical waveguide  105  and is taken out by the optical fiber  114 . 
     Hereinafter, an optical inspection method according to an embodiment of the present invention will be described with reference to  FIG. 2 . First, in a first step S 101 , continuous light emitted from a light source  113  is made incident via a first optical waveguide  104  to an optical modulator  102 . Next, in a second step S 102 , modulated light modulated by an optical modulator  110  is made incident to a photodiode  103 . As described above, a modulated electrical signal is outputted from the photodiode  103 , to which the modulated light is made incident, to the optical modulator  102 . The optical modulator  102  modulates the inputted continuous light by the modulated electrical signal received by the photodiode  103  and outputs the modulated continuous light. 
     Thereafter, in a third step S 103 , the modulated light outputted from the optical modulator  102  is evaluated. By conducting evaluation such as comparison between the modulated light outputted from the optical modulator  102  and taken out from the optical fiber  114  and the modulated light outputted from the optical modulator  110  (inputted to the photodiode  103 ), inspection of the optical modulator  102  at a wafer level can be implemented. 
     As described above, according to the present embodiment, without requiring a probe card which is excellent in high frequency characteristics and a high frequency cable for connecting a probe card outside a wafer and a device on the wafer, the inspection of the optical modulator  102  at the wafer level can be implemented. 
     For example, when with respect to power of the light inputted to the photodiode  103 , a voltage of the electrical signal which the photodiode  103  outputs is measured, change of the voltage occurs as shown in  FIG. 3 . As shown in  FIG. 3 , when light of approximately one mW is inputted to the photodiode  103 , a voltage of approximately 0.15 V is generated. 
     On the other hand, a result of measuring relationship between a voltage of an electrical signal inputted (applied) to the optical modulator  102  and light outputted from the optical modulator  102  is shown in  FIG. 4 . As shown in  FIG. 4 , it is seen therefrom that when an electrical signal of a voltage of approximately 0.2 V is applied to the optical modulator  102 , change of approximately 0.5 dB occurs in the outputted light. 
     Next, a light pulse (modulated light) of one mW at peak power is inputted to the photodiode  103  connected to the optical modulator  102  by the wire  107 , and as a result of this, a light output waveform of the modulated light outputted from the optical modulator  102  is shown in  FIG. 5 . As is predicted from the results shown in  FIG. 3  and  FIG. 4 , the light modulated signal of approximately 0.5 dB is outputted, and it is indicated that the optical inspection circuit according to the present embodiment enables characteristic inspection of the optical modulator  102 . 
     Next, another optical inspection circuit according to the embodiment of the present invention will be described with reference to  FIG. 6 . This optical inspection circuit includes an optical modulator  102  formed on a substrate  101  and a photodiode  103  formed on the substrate  101  in the vicinity of the optical modulator  102 . In addition, optically connected to the optical modulator  102  are a first optical waveguide  104  and a second optical waveguide  105 . In addition, optically connected to the photodiode  103  is a third optical waveguide  106 . In addition, the optical modulator  102  and the photodiode  103  are electrically connected by a wire  107 . The configuration of these is similar to that of the optical inspection circuit described with reference to  FIG. 1 . 
     This optical inspection circuit further includes an optical distributor  115  which distributes inputted signal light to the first optical waveguide  104  and the third optical waveguide  106 . In addition, the optical inspection circuit includes a wavelength filter  116  which takes out light having a predetermined wavelength from light outputted from the optical modulator  102 . The optical distributor  115  and the wavelength filter  116  are formed on the substrate  101 . In addition, each of the optical distributor  115  and the wavelength filter  116  comprises an optical waveguide having a core comprising a semiconductor. 
     The optical distributor  115  is to distribute inputted light power to a plurality of optical waveguides and can be configured of, for example, a Y-branch circuit. In addition, the optical distributor  115  can also be configured of a multi-mode interferometer. In addition, the optical distributor  115  can also be configured of a directional coupler. The wavelength filter  116  can be configured of a two-output optical circuit having wavelength dependence. The wavelength filter  116  can be configured of, for example, an array diffraction grating, an asymmetric Mach-Zehnder interferometer, a directional coupler, or the like. 
     In this optical inspection circuit, continuous light and modulated light are multiplexed and the multiplexed light is inputted to the optical distributor  115 . The optical distributor  115  distributes the multiplexed continuous light and the modulated light. Accordingly, the multiplexed continuous light and the modulated light are inputted to both of the optical modulator  102  and the photodiode  103 . In this configuration, a modulated electrical signal generated by the photodiode  103  by the modulated light in the multiplexed light is used, and the optical modulator  102  modulates inputted light. The multiplexed continuous light and the modulated light are inputted to the optical modulator  102 , and of this inputted light, the continuous light is targeted for the modulation and the modulated light becomes noise. The light corresponding to the noise is eliminated by the wavelength filter  116 . This optical inspection circuit has excellent effects which allow a number of the optical waveguides for inputting to be reduced to one. 
     Next, still another optical inspection circuit according to the embodiment of the present invention will be described with reference to  FIG. 7 . This optical inspection circuit includes a plurality of optical modulators  102   a ,  102   b , and  102   c  formed on a substrate  101  and a photodiode  103  formed on the substrate  101  in the vicinity of the optical modulator  102   a ,  102   b ,  102   c.    
     Optically connected to the optical modulator  102   a  are a first optical waveguide  104   a  and a second optical waveguide  105   a . Optically connected to the optical modulator  102   b  are a first optical waveguide  104   b  and a second optical waveguide  105   b . Optically connected to the optical modulator  102   c  are a first optical waveguide  104   c  and a second optical waveguide  105   c . In addition, optically connected to the photodiode  103  is a third optical waveguide  106 . 
     In addition, each of the optical modulators  102   a ,  102   b , and  102   c  and the photodiode  103  are electrically connected by a wire  107   a . The wire  107   a  can be formed by, for example, wire bonding. In addition, the wire  107   a  can also be configured of the heretofore known multilayer wiring structure formed in a surface layer of the substrate  101 . 
     This optical inspection circuit has excellent effects which allow the plurality of optical modulators  102   a ,  102   b , and  102   c  to be inspected by the modulated electrical signal generated by one photodiode  103 . 
     Next, yet another optical inspection circuit according to the embodiment of the present invention will be described with reference to  FIG. 8 . This optical inspection circuit includes an optical modulator  102  formed on a substrate  101  and a plurality of photodiodes  103   a ,  103   b , and  103   c  formed on the substrate  101  in the vicinity of the optical modulator  102 . 
     In addition, optically connected to the optical modulator  102  are a first optical waveguide  104  and a second optical waveguide  105 . In addition, optically connected to the photodiode  103   a  is a third optical waveguide  106   a . In addition, optically connected to the photodiode  103   b  is a third optical waveguide  106   b . In addition, optically connected to the photodiode  103   c  is a third optical waveguide  106   c.    
     In addition, the optical modulator  102  and the photodiode  103  are electrically connected by a wire  107   b . The photodiodes  103   a ,  103   b , and  103   c  whose number is plural are series-connected to the optical modulator  102  by the wire  107   b . The wire  107   b  can be formed by, for example, wire bonding. In addition, the wire  107   b  can also be configured of the heretofore known multilayer wiring structure formed in a surface layer of the substrate  101 . By employing this optical inspection circuit, the plurality of photodiodes  103   a ,  103   b , and  103   c  is used, and a modulated electrical signal having further large voltage can be thereby generated, and thus, the optical inspection circuit has excellent effects which enables inspection of a characteristic, which requires a large voltage, among modulation characteristics of the optical modulator  102 . 
     As described hereinbefore, according to embodiments of the present invention, since on the substrate on which the optical modulator is formed, the photodiode is formed, and the optical modulator and the photodiode are electrically connected by the wire, inspection of an optical circuit such as an optical modulator at a wafer level can be further easily and further inexpensively implemented. 
     Note that the present invention is not limited to the above-described embodiment and it is apparent that many modifications and combinations can be implemented by those having ordinary skill in the art without departing from the spirit and scope of technical idea of the present invention. 
     REFERENCE SIGNS LIST 
     
         
         
           
               101  Substrate 
               102  Optical modulator 
               103  Photodiode 
               104  First optical waveguide 
               105  Second optical waveguide 
               106  Third optical waveguide 
               107  Wire 
               109  Optical fiber 
               110  Optical modulator 
               111  Light source 
               112  Optical fiber 
               113  Light source 
               114  Optical fiber 
               121 ,  122 ,  123  Electrode pad 
               131 ,  132 ,  133  Electrode pad 
               171  Signal wire 
               172 ,  173  Grounding wire