Patent Publication Number: US-2004057684-A1

Title: Optical waveguide and method of manufacturing the same

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to an optical waveguide constituting an optical integrated circuit device and a method of manufacturing the same.  
       [0003] 2. Description of the Prior Art  
       [0004] In recent years, the higher integration density and the higher density of the optical integrated circuit device are required. Accompanying with this, in order to enhance a propagation efficiency of the light, it is required to increase difference in the refractive index between the core layer, which mainly propagates the light, and the cladding layer, which covers the core layer, constituting the optical waveguide. In particular, 1.55 μm is used as the wavelength of the light in the communication field. At present, in this wavelength bandwidth, the film in which Ge or P is added to SiO 2  is employed as the core layer and also the SiO 2  film is employed as the cladding layer.  
       [0005] However, according to the current technology, difference in the refractive index between the core layer and the cladding layer is very small such as 0.04, and the permeation of the propagated light into the cladding layer and the loss are still large. Also, there was such a problem that, since the refractive index itself is small, patterns of the optical devices such as the resonator, etc. are increased in size.  
       SUMMARY OF THE INVENTION  
       [0006] It is an object of the present invention to provide an optical waveguide capable of increasing a refractive index itself of a core layer and increasing difference in the refractive index between the core layer and a cladding layer, and a method of manufacturing the same.  
       [0007] Advantages attained based on the above configuration of the present invention will be explained hereunder.  
       [0008] In the present invention, the silicon nitride film serving as the core layer is formed by plasmanizing the gas mixture containing methylsilane and at least any one of nitrogen (N 2 ) or ammonia (NH 3 ) to react.  
       [0009] According to the experiment, the silicon nitride film with the large refractive index such as about 2 can be formed by the above gas mixture. In addition, the film having the good refractive index distribution and the good film thickness distribution could be obtained. Since the core layer has the large refractive index by employing this silicon nitride film as the core layer, patterns of the optical devices such as the resonator, etc. could be reduced in size. Further, since the film having the good refractive index distribution and the good film thickness distribution can be obtained, uniformization of device characteristics in the wafer could be achieved.  
       [0010] Also, the silicon oxy-nitride film serving as the core layer or the cladding layer is formed by plasmanizing the gas mixture containing any one of methylsilane, alkyl compound having the siloxane bond, or alkyl compound having the alkoxy bond, dinitrogen monoxide (N 2 O), and at least any one of the nitrogen (N 2 ) or the ammonia (NH 3 ) to react.  
       [0011] According to the experiment, the refractive index of the silicon oxy-nitride film to be formed could be adjusted in the wide range from about 2 to about 1.4 by controlling the flow rate of dinitrogen monoxide (N 2 O) in the gas mixture having the above constitution. Therefore, the silicon oxy-nitride film can be employed as the core layer or the cladding layer.  
       [0012] Also, the silicon oxide film serving as the cladding layer is formed by plasmanizing the gas mixture containing methylsilane and dinitrogen monoxide (N 2 O) to react.  
       [0013] According to the experiment, the silicon oxide film having the small refractive index of about 1.4 could be formed by using the above gas mixture. Also, the film having the good refractive index distribution and the good film thickness distribution could be obtained.  
       [0014] In addition, if the core layer is formed of the above silicon nitride film or the silicon oxy-nitride film and the cladding layer is formed of the above silicon oxide film or the silicon oxy-nitride film, difference in the refractive index between the core layer and the cladding layer can be increased up to about 0.5 to 0.6. It results in effective suppression of the permeation of the light propagated through the core layer into the cladding layer and the loss.  
       [0015] Also, by a combination of above manufacturing methods, both the core layer and the cladding layer can be formed by only the plasma CVD method. Therefore, control of the film thickness can be facilitated. It leads to not only the reduction in size of the patterns of the optical devices due to the higher refractive index but also the higher integration density and the higher density of the optical integrated circuit device. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0016]FIG. 1 is a side view showing a configuration of a plasma CVD apparatus employed in optical waveguide manufacturing methods as embodiments of the present invention;  
     [0017]FIG. 2 is a table showing combinations of particularly effective gases constituting a film forming gas employed in the optical waveguide manufacturing method as the embodiment of the present invention;  
     [0018]FIG. 3 is a graph showing a refractive index in-plane distribution and a film thickness in-plane distribution of a silicon nitride film formed under the film forming conditions I for the optical waveguide manufacturing method according to a first example of the present invention;  
     [0019]FIG. 4 is a graph showing a refractive index in-plane distribution and a film thickness in-plane distribution of a silicon nitride film formed under the film forming conditions II for the optical waveguide manufacturing method according to a second example of the present invention; and  
     [0020]FIG. 5A is a perspective view showing a configuration of an optical integrated circuit device round a ring resonator as the embodiment, and FIG. 5B is a sectional view taken along a I-I line in FIG. 5A. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0021] Embodiments of the present invention will be explained with reference to the drawings hereinafter.  
     [0022] (Explanation of a Plasma CVD Apparatus Used in a Film Forming Method as an Embodiment of the Present Invention)  
     [0023]FIG. 1 is a side view showing a configuration of a parallel-plate plasma CVD apparatus  101  employed in an optical waveguide manufacturing method according to the embodiment of the present invention.  
     [0024] This plasma CVD apparatus  101  consists of a film forming portion  101 A as a portion at which a silicon-containing insulating film serving as the core layer or the cladding layer is formed on a substrate  21  subject to film-forming by the plasma gas, and a film-forming gas supplying portion  101 B having a plurality of gas supply sources constituting film forming gases.  
     [0025] As shown in FIG. 1, the film forming portion  101 A has a chamber  1  whose inner pressure can be lowered, and this chamber  1  is connected to an exhausting apparatus  6  via an exhaust piping  4 . An opening/closing valve  5  for controlling communication/uncommunication between the chamber  1  and the exhausting apparatus  6  is provided to the middle of the exhaust piping  4 . A pressure measuring means such as a vacuum gauge (not shown) for monitoring the pressure in the chamber  1  is provided to the chamber  1 .  
     [0026] A pair of opposing electrodes, i.e., an upper electrode  2  and a lower electrode  3 , are provided to the inside of the chamber  1 . A high-frequency power supply (RF power supply)  7  for supplying a high-frequency power of a frequency 13.56 MHz is connected to the upper electrode  2 . A low-frequency power supply  8  for supplying a low-frequency power of a frequency 380 kHz is connected to the lower electrode  3 . The film forming gas is plasmanized by supplying the high-frequency power to the upper electrode  2  from at least the high-frequency power supply  7  out of these power supplies  7 ,  8 . The upper electrode  2 , the lower electrode  3 , and the power supplies  7 ,  8  constitute a plasma generating means that plasmanizes the film forming gas.  
     [0027] In this case, the low-frequency power having not only the frequency of 380 kHz but also the frequency of 50 kHz to 1 MHz may be applied to the lower electrode  3 . Also, the high-frequency power having not only the frequency of 13.56 MHz but also the frequency of 1 MHz or more may be applied to the upper electrode  2  that opposes to the lower electrode  3 . In addition, in FIG. 1, the high-frequency power supply is connected to the upper electrode  2  and the low-frequency power supply is connected to the lower electrode  3 . However, the high-frequency power supply  7  is not limited to the connection to the upper electrode  2 , and also the low-frequency power supply  8  is not limited to the connection to the lower electrode  3 . The high-frequency power supply  7  may be connected to any one of the upper electrode  2  and also the low-frequency power supply  8  may be connected to other electrode. Even by this constitution, the object of the present invention can be attained.  
     [0028] The upper electrode  2  is also used as a distributor of the film forming gas. A plurality of through holes are formed in the upper electrode  2 , and opening portions of the through holes on the opposing surface to the lower electrode  3  act as discharge ports (introduction ports) of the film forming gas. The discharge ports are connected to the film-forming gas supplying portion  101 B via a piping  9   a . Also, if the case may be, a heater (not shown) may be provided to the upper electrode  2 . This is because, if the upper electrode  2  is heated around the temperature of almost 100° C. during the film formation, particles made of the reaction product of the film forming gas can be prevented from adhering to the upper electrode  2 .  
     [0029] The lower electrode  3  is also used as a loading table of the substrate  21  subject to film-forming. Also, the lower electrode  3  has a heater  12  for heating the substrate  21  subject to film-forming loaded on the loading table.  
     [0030] In the film-forming gas supplying portion  101 B, a supply source of methylsilane represented by a general formula SiH n (CH 3 ) 4−n  (n=0 to 3), a supply source of alkyl compound having the siloxane bond such as hexamethyldisiloxane (HMDSO: (CH 3 ) 3 Si—O—Si(CH 3 ) 3 ), or the like, a supply source of alkyl compound having the alkoxy bond, a supply source of ammonia (NH 3 ), a supply source of dinitrogen monoxide (N 2 O), a supply source of at least any one of He or Ar (rare gas), and a supply source of nitrogen (N 2 ) as an inert gas are provided.  
     [0031] These gases are supplied appropriately to the inside of the chamber  1  of the film forming portion  101 A via branch pipings  9   b  to  9   h  and a piping  9   a  to which these branch pipings  9   b  to  9   h  are connected. Flow-rate adjusting means  11   a  to  11   g  and opening/closing means  10   b  to  10   u  for controlling communication/uncommunication of the branch pipings  9   b  to  9   h  are provided in the middle of the branch pipings  9   b  to  9   h . Also, an opening/closing means  10   a  for executing close/open of the piping  9   a  is provided in the middle of the piping  9   a . In addition, in order to purge residual gases in the branch pipings  9   b  to  9   f  by flowing the N 2  gas through them, opening/closing means  10   p  to  10   u  are provided in order to control communication/uncommunication between the branch piping  9   h  connected to the N 2  gas supply source and other branch pipings  9   b  to  9   g . In this case, the N 2  gas is also used to purge residual gases in the piping  9   a  and the chamber  1  in addition to the branch pipings  9   b  to  9   g . Also, the N 2  gas is also used as the film forming gas.  
     [0032] The above film forming apparatus  101  is provided with the supply source of methylsilane represented by a general formula SiH n (CH 3 ) 4−n  (n=0 to 3), the supply source of alkyl compound having the siloxane bond such as hexamethyldisiloxane (HMDSO), or the like, the supply source of alkyl compound having the alkoxy bond, the supply source of ammonia (NH 3 ), the supply source of dinitrogen monoxide (N 2 O), the supply source of at least any one of He or Ar (rare gas), and the supply source of nitrogen (N 2 ) as an inert gas, and also plasma generating means  2 ,  3 ,  7 ,  8  for plasmanizing the film forming gas.  
     [0033] As a result, the core layer made of a silicon nitride film or a silicon oxy-nitride film, which has a high refractive index, and the cladding layer made of a silicon oxide film or the silicon oxy-nitride film, which has a low refractive index, can be formed employing the plasma CVD method described later.  
     [0034] Next, an explanation will be performed hereunder with respect to methylsilane, alkyl compound having the siloxane bond, and alkyl compound having the alkoxy bond, which are as the film forming gas employed in the present invention.  
     [0035] As typical examples, followings may be employed.  
     [0036] (i) methylsilane (SiH n (CH 3 ) 4−n :n=0 to 3)  
     [0037] monomethylsilane (SiH 3 (CH 3 ))  
     [0038] dimethylsilane (SiH 2 (CH 3 ) 2 )  
     [0039] trimethylsilane (SiH(CH 3 ) 3 )  
     [0040] tetramethylsilane (Si(CH 3 ) 4 )  
     [0041] (ii) Alkyl Compound Having the Siloxane Bond  
     [0042] hexamethyldisiloxane (HMDSO: (CH 3 ) 3 Si—O—Si(CH 3 ) 3 )  
     [0043] octamethylcyclotetrasiloxane (OMCTS)  
                 
 
     [0044] octamethyltrisiloxane (OMTS)  
                 
 
     [0045] (iii) Alkyl Compound Having the Alkoxy Bond  
     [0046] dimethyldimethoxysilane (Si(CH 3 ) 2 (OCH 3 ) 2 )  
     [0047] dimethyldiethoxysilane (Si(CH 3 ) 2 (OC 2 H 5 ) 2 )  
     [0048] trimethoxysilane (TMS: SiH(OCH 3 ) 3 )  
     [0049] (Explanation of the Film Forming Method as the Embodiment of the Present Invention)  
     [0050] Next, particularly effective combination of gases constituting the film forming gas employed in the optical waveguide manufacturing method as the embodiment of the present invention will be explained with reference to FIG. 2 hereunder.  
     [0051]FIG. 2 is a table showing respective combinations of gases constituting the film forming gas to form the silicon nitride film (SiN film), the silicon oxy-nitride film (SiON film), and the silicon oxide film (SiO film), which serves as the core layer or the cladding layer. In the following, particularly effective constitutions of the film forming gas will be listed concretely in compliance with the indication of formed films in FIG. 2.  
     [0052] (i) Constitutive Gases of the Film Forming Gas to Form the Silicon Nitride Film (SiN Film)  
     [0053] (1) methylsilane/N 2    
     [0054] (2) methylsilane/N 2 /Ar or He  
     [0055] (3) methylsilane/NH 3    
     [0056] (4) methylsilane/NH 3 /Ar or He  
     [0057] (5) methylsilane/N 2 /NH 3    
     [0058] (6) methylsilane/N 2 /NH 3 /Ar or He  
     [0059] (ii) Constitutive Gases of the Film Forming Gas to Form the Silicon Oxy-Nitride Film (SiON Film)  
     [0060] (7) methylsilane/N 2 /N 2 O  
     [0061] (8) methylsilane/N 2 /N 2 O/Ar or He  
     [0062] (9) methylsilane/NH 3 /N 2 O  
     [0063] (10) methylsilane/NH 3 /N 2 O/Ar or He  
     [0064] (11) methylsilane/N 2 /NH 3 /N 2 O  
     [0065] (12) methylsilane/N 2 /NH 3 /N 2 O/Ar or He  
     [0066] (13) siloxane/N 2 /NH 3 /N 2 O  
     [0067] (14) siloxane/N 2 /NH 3 /N 2 O/Ar or He  
     [0068] (15) alkoxy compound/N 2 /NH 3 /N 2 O  
     [0069] (16) alkoxy compound/N 2 /NH 3 /N 2 O/Ar or He  
     [0070] In addition to the constitutive gases given in above (7) to (16), oxygen (O 2 ) may be added.  
     [0071] (iii) A Constitutive Gas of the Film Forming Gas to Form the Silicon Oxide Film (SiO Film)  
     [0072] (17) methylsilane/N 2 O  
     [0073] In addition to the constitutive gases given in above (17), oxygen (O 2 ) may be added.  
     [0074] According to the experiment made by the inventors of this application, it was found that the refractive index can be adjusted by controlling a flow rate of N 2 O contained in the above film forming gas or controlling a flow rate of Ar or He.  
     [0075] In particular, when the SiON film is formed, the refractive index could be controlled to about 1.4 to 2 by adjusting the flow rate of N 2 O in the film forming gases given in above (7) to (16). In more detail, the refractive index became low if the flow rate of N 2 O was increased, while the refractive index became high if the flow rate of N 2 O was reduced. The reason for this may be supposed as follows. That is, since the electron density in the film is reduced if the oxygen is increased, the refractive index becomes low. In contrast, since the electron density in the film is increased if the oxygen is lowered, the refractive index becomes high. For this reason, the SiON film can be employed as the core layer and the cladding layer.  
     [0076] On the other hand, the silicon nitride film having the refractive index of about 2 was obtained. Therefore, this film can be employed as the core layer. The silicon oxide film having the refractive index of about 1.4 was obtained. Therefore, this film can be employed as the cladding layer.  
     [0077] Further, if the silicon nitride film, the silicon oxy-nitride film, or the silicon oxide film is brought into contact with the N 2 O or O 2  plasma, the film quality can be improved. Particularly, if such plasma process is applied to the silicon oxy-nitride film or the silicon oxide film serving as the cladding layer, the lower refractive index can be attained.  
     [0078] Next, examined results will be explained hereunder with respect to a refractive index in-plane distribution and a film thickness in-plane distribution of the silicon-containing insulating film, which is formed by the optical waveguide manufacturing method of the present invention by using the above plasma CVD apparatus.  
     (1) FIRST EXAMPLE  
     [0079] As the sample, the silicon nitride film was formed on the substrate subject to film-forming. This silicon nitride film was formed under following film forming conditions by the plasma CVD method.  
     [0080] (Film Forming Conditions I)  
     [0081] (i) Film Forming Gas Conditions  
     [0082] tetramethylsilane flow rate: 50 sccm  
     [0083] N 2  flow rate: 100 sccm  
     [0084] NH 3  flow rate: 100 sccm  
     [0085] Ar flow rate: 300 sccm  
     [0086] gas pressure: 1.1 Torr  
     [0087] (ii) Plasmanizing Conditions  
     [0088] high-frequency power (13.56 MHz) PRF: 100 W  
     [0089] low-frequency power (380 kHz) PLF: 250 W  
     [0090] (iii) Substrate heating temperature: 300° C.  
     [0091] (iv) silicon nitride film formed (see FIG. 3)  
     [0092] film thickness (in-plane): 2.5739 μm±30 nm  
     [0093] refractive index (in-plane): 2.0664±0.0004  
     [0094] (measure wavelength: 1.55 μm)  
     [0095] As described above, according to the above experiment, the silicon nitride film having the large refractive index of about 2 could be formed by using the above gas mixture, and also the film having a good refractive index distribution and a good film thickness distribution could be obtained. If this silicon nitride film is employed as the core layer, the core layer can have the high refractive index, so that patterns of the optical devices such as the resonator, etc. can be reduced in size. In addition, since the film having the good refractive index distribution and the good film thickness distribution can be obtained, uniformization of device characteristics in the wafer can be achieved.  
     (2) SECOND EXAMPLE  
     [0096] As the sample, like the first example, the silicon oxide film was formed on the substrate subject to film-forming. This silicon oxide film was formed under following film forming conditions by the plasma CVD method.  
     [0097] (Film Forming Conditions II)  
     [0098] (i) Film Forming Gas Conditions  
     [0099] tetramethylsilane flow rate: 50 sccm  
     [0100] N 2 O flow rate: 2000 sccm  
     [0101] gas pressure: 1.0 Torr  
     [0102] (ii) Plasmanizing Conditions  
     [0103] high-frequency power (13.56 MHz) PRF: 150 W  
     [0104] low-frequency power (380 kHz) PLF: 300 W  
     [0105] (iii) Substrate heating temperature: 300° C.  
     [0106] (iv) Silicon oxide film formed (see FIG. 4)  
     [0107] film thickness (in-plane): 3.00326 μm±30 nm  
     [0108] refractive index (in-plane): 1.4642+0.0006  
     [0109] (measure wavelength: 1.55 μm)  
     [0110] As described above, according to the above experiment, the silicon oxide film having the small refractive index of about 1.4 could be formed by using the above gas mixture, and also the film having the good refractive index distribution and the good film thickness distribution could be obtained.  
     [0111] As described above, if the silicon oxide film or the silicon oxy-nitride film of the present invention is employed as the cladding layer and the silicon nitride film or the silicon oxy-nitride film of the present invention is employed as the core layer, difference in the refractive index between the core layer and the cladding layer can be set to about 0.5 to 0.6. It causes a remarkable reduction of the permeation of the light propagated through the core layer into the cladding layer and the loss.  
     [0112] Also, if above manufacturing methods are combined with each other, both the core layer and the cladding layer can be formed by the plasma CVD method. It causes a facilitated control of the film thickness. It results in not only the reduction in size of the patterns of the optical devices due to the higher refractive index but also the higher integration density and the higher density of the optical integrated circuit device.  
     [0113] (Application to the Optical Waveguide)  
     [0114] Next, an explanation will be performed hereunder with reference to FIGS. 5A and 5B about an example in which the above manufacturing method is applied to the optical waveguide.  
     [0115]FIG. 5A is a perspective view showing a configuration of an optical integrated circuit device round a ring resonator. FIG. 5B is a sectional view taken along a I-I line in FIG. 5A.  
     [0116] As shown in FIG. 5A, bus line waveguides  22   a ,  22   b , which intersect with each other, and a ring resonator  23  is provided on the silicon substrate (substrate subject to film-forming)  21 . The ring resonator  23  is formed near the intersected portion by shaping the stripe-like waveguide with a width of about 1.5 μm into a ring shape whose diameter is about 20 μm. As shown in FIG. 5B, surrounding portions of the bus line waveguides  22   a ,  22   b  are covered completely with a lower cladding layer  24 . The ring resonator  23  consists of a core layer  23   a  formed on the lower cladding layer  24 , and an upper cladding layer  23   b  laminated on the core layer  23   a . In FIG. 5A, the lower cladding layer  24  is omitted.  
     [0117] A part of a circumference of the ring resonator  23  overlaps with the bus line waveguides  22   a ,  22   b , and a resonance wavelength (λ) is decided by  
       n×λ= 2π r×δ.    
     [0118] Where r is a radius of the ring resonator  23 , δ is the refractive index of the ring core layer  23   a , and n is a positive integer.  
     [0119] If the ring resonator  23  is set to resonate with the light having a wavelength λ2, for example, as shown in FIG. 5A, the wavelength λ2 out of lights (λ0, λ1, λ2, λ3, . . . , λn), which are input from an input port and whose frequencies are multiplied, is output from a drop port by the ring resonator  23 , and also the lights (λ0, λ1, λ3, . . . , λn), which have other frequencies, are output from a through port.  
     [0120] In the above configuration, the bus line waveguides  22   a ,  22   b  are formed of the silicon nitride film or the silicon oxy-nitride film, which has the refractive index of about 2 and has a film thickness of about 0.5 to 0.7 μm, and are formed by using the plasma-enhanced CVD method. Also, the ring core layer  23   a  is formed of the silicon nitride film or the silicon oxy-nitride film, which has the refractive index of about 2 and has the film thickness of about 1 to 2 μm, and is formed by using the plasma-enhanced CVD method. In addition, the upper cladding layer  23   b  is formed of the silicon oxide film or the silicon oxy-nitride film, which has the refractive index of about 1.4 and has the film thickness of about 2 to 3 μm, and is formed by using the plasma-enhanced CVD method. The lower cladding layer  24  is formed of the silicon oxide film or the silicon oxy-nitride film, which has the refractive index of about 1.4 and has the film thickness of about 8 μm, and is formed by using the plasma-enhanced CVD method.  
     [0121] According to the ring resonator  23 , since the silicon nitride film or the silicon oxy-nitride film formed by the plasma-enhanced CVD method is employed as the bus line waveguides  22   a ,  22   b  and the core layer  23   a  and the silicon oxide film or the silicon oxy-nitride film is employed as the cladding layers  23   b ,  24 , difference in the refractive index between the core layer and the cladding layer can be increased up to about 0.5 to 0.6. It can cause a remarkable reduction of the permeation of the light propagated through the core layer into the cladding layer and the loss.  
     [0122] Also, uniformization of the optical device characteristics can be achieved by reduction of variation in the refractive index distribution and the film thickness distribution in the bus line waveguides  22   a ,  22   b , the core layer  23   a , and the cladding layers  23   b ,  24 .  
     [0123] In addition, miniaturization of the patterns of the bus line waveguides  22   a ,  22   b  and the ring resonator  23  can be attained by increase of the refractive indices of the bus line waveguides  22   a ,  22   b  and the core layer  23   a . As a result, the higher integration density and the higher density of the optical integrated circuit device can be attained.  
     [0124] Further, since both the core layer and the cladding layer can be formed by the CVD method, control of the film thickness of respective layers can be facilitated. As a result, not only the reduction in size of the patterns of the optical devices due to the higher refractive index but also the higher integration density and the higher density of the optical integrated circuit device can be attained.  
     [0125] With the above, the present invention is explained in detail based on the embodiments. But a scope of the present invention is not limited to the examples that are concretely shown in the above embodiments. It should be interpreted that variation and modification of the above embodiments, which do not depart from a gist of the present invention, may be contained in the scope of the present invention.  
     [0126] For example, in the first example, the gas mixture consisting of tetramethylsilane (Si(CH 3 ) 4 ), N 2 , NH 3 , and Ar is employed as the film forming gas applied to the silicon nitride film (SiN film) serving as the core layer, but the film forming gas is not limited to this gas mixture. The combinations shown in FIG. 2 may be employed. Also, in place of tetramethylsilane (Si(CH 3 ) 4 ), other methylsilane, e.g., any one of monomethylsilane (SiH 3 (CH 3 )), dimethylsilane (SiH 2 (CH 3 ) 2 ), or trimethylsilane (SiH(CH 3 ) 3 ) may be employed.  
     [0127] Also, in the second example, the gas mixture consisting of tetramethylsilane and N 2 O is employed as the film forming gas applied to the silicon oxide film (SiO film) serving as the cladding layer, but the film forming gas is not limited to this gas mixture. The combinations shown in FIG. 2 may be employed. Also, in place of tetramethylsilane (Si(CH 3 ) 4 ), other methylsilane, e.g., any one of monomethylsilane (SiH 3 (CH 3 )), dimethylsilane (SiH 2 (CH 3 )  2 ), or trimethylsilane (SiH(CH 3 ) 3 ) may be employed.  
     [0128] In addition, as the film forming gas applied to the silicon oxy-nitride film (SiON film) serving as the core layer or the cladding layer, the combinations shown in FIG. 2 may be employed. In this case, any one of monomethylsilane (SiH 3 (CH 3 )), dimethylsilane (SiH 2 (CH 3 )  2 ), trimethylsilane (SiH(CH 3 ) 3 ), or tetramethylsilane (Si(CH 3 ) 4 ), for example, may be employed as methylsilane.  
     [0129] Further, in place of methylsilane, alkyl compound having the siloxane bond, e.g., hexamethyldisiloxane (HMDSO: (CH 3 ) 3 Si—O—Si(CH 3 ) 3 ), octamethylcyclotetrasiloxane (OMCTS), or octamethyltrisiloxane (OMTS) may be employed.  
     [0130] Furthermore, in place of methylsilane, alkyl compound having the alkoxy bond, e.g., dimethyldimethoxysilane (Si(CH 3 ) 2 (OCH 3 ) 2 ), dimethyldiethoxysilane (Si(CH 3 ) 2 (OC 2 H 5 ) 2 ), or trimethoxysilane (TMS: SiH(OCH 3 ) 3 ) may be employed.  
     [0131] Moreover, the present invention is applied to the optical waveguide through which the signal light having the wavelength of 1.55 μm is propagated, and the refractive index is adjusted to the range between 1.4 and 2. But the present invention is not limited to the above. If the refractive index is adjusted to the appropriate range other than the range between 1.4 and 2, the present invention can be applied to the optical waveguide through which the light having the practical wavelength is propagated.  
     [0132] As described above, according to the present invention, the silicon nitride film serving as the core layer is formed by plasmanizing the gas mixture containing methylsilane and at least any one of nitrogen (N 2 ) or ammonia (NH 3 ) to react.  
     [0133] Therefore, since the core layer has the large refractive index, patterns of the optical devices such as the resonator, etc. can be reduced in size. In addition, since the film having the good refractive index distribution and the good film thickness distribution can be obtained, uniformization of device characteristics in the wafer can be achieved.  
     [0134] Also, the silicon oxy-nitride film serving as the core layer or the cladding layer is formed by plasmanizing the gas mixture containing any one of methylsilane, alkyl compound having the siloxane bond, or alkyl compound having the alkoxy bond, dinitrogen monoxide (N 2 O), and at least any one of the nitrogen (N 2 ) or the ammonia (NH 3 ) to react. The refractive index of the silicon oxy-nitride film to be formed can be adjusted in the wide range from about 2 to about 1.4 by controlling the flow rate of dinitrogen monoxide (N 2 O) in the gas mixture having the above constitution. In addition, the film having the good refractive index distribution and the good film thickness distribution can be obtained.  
     [0135] Also, the silicon oxide film serving as the cladding layer is formed by plasmanizing the gas mixture containing methylsilane and dinitrogen monoxide (N 2 O) to react. The silicon oxide film having the small refractive index of about 1.4 can be formed by using the above gas mixture, and also the film having the good refractive index distribution and the good film thickness distribution can be obtained.  
     [0136] In addition, since the above silicon nitride film or the silicon oxy-nitride film is employed as the core layer and the above silicon oxide film or the silicon oxy-nitride film is employed as the cladding layer, difference in the refractive index between the core layer and the cladding layer can be increased up to about 0.5 to 0.6. Therefore, the permeation of the light propagated through the core layer into the cladding layer and the loss can be reduced largely.  
     [0137] Also, if above manufacturing methods are used in combination, both the core layer and the cladding layer can be formed by the plasma CVD method. Therefore, control of the film thickness can be facilitated. As a result, not only the reduction in size of the patterns of the optical devices due to the higher refractive index but also the higher integration density and the higher density of the optical integrated circuit device can be attained.