JP2003337311A - Optical device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical device, particularly, an optical waveguide device, in which characteristics of a light signal are modulated or changed in accordance with an applied electric field. <P>SOLUTION: Conventionally, in such devices, such as, for example, a Mach- Zehnder modulator, DC drift problem, as are well known within the art, must be surmounted if the optical device is to meet minimum performance criteria. Suitably, a layer of an oxide of silicon, preferably substantially, free of metallic impurities, where the ratio of oxygen to silicon is greater than 2 and is preferably greater than or equal to 2.2 is provided. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device and its manufacturing method, and more particularly to an optical waveguide device and its manufacturing method.

[0002]

BACKGROUND OF THE INVENTION Conventionally, an RF electric field is applied to a waveguide region in an optical device to change the refractive index of the waveguide, modulating the intensity and / or phase of light traveling within the waveguide.

Usually, this modulated RF electric field is applied through an electrode formed on a buffer layer which covers the surface of a substrate on which a waveguide has been previously formed. The buffer layer is typically composed of a dielectric film having a lower refractive index than the waveguide.
One of the functions of the buffer layer is to reduce light loss due to absorption in the metal electrode. Another function of the buffer layer is to obtain velocity matching between the microwave (RF) propagating in the electrode and the light propagating in the waveguide.

For example, lithium niobate (LiNbO ₃ ).
Examples of the optical waveguide device using the electro-optic crystal substrate such as the above are a Mach-Zehnder interferometric modulator, an electro-optic polarization (polarization) control device, a variable optical attenuator, and an electro-optic switch.

Specifically, in the Mach-Zehnder modulator, the relative optical path lengths of two signals derived from a single optical signal are changed as a result of the change in the refractive index of the corresponding waveguide according to the applied RF electric field. Therefore, upon recombination, constructive or destructive interference occurs depending on the relative phase of the two optical signals.

Here, the preferable phase difference state can be changed by temperature and time, but is controlled by simultaneously applying a DC bias voltage through an appropriate electrode structure. Requiring a DC bias voltage to ensure stable operation of the device even in a service atmosphere is a well known technique in the art. Therefore, many optical devices use a DC bias voltage feedback control circuit to maintain a stable optical output signal.

Fluctuations in DC bias voltage as a result of this DC bias voltage feedback control are known in the art as DC drift of optical devices. In fact, performance specifications imposed on such optical devices include that the applied DC bias voltage must be within predetermined limits for 25 years.

US Pat. No. 5, assigned to Fujitsu Limited
No. 404,412 discloses an optical device having reduced DC drift, that is, an optical device having a configuration in which a DC bias voltage is maintained within a predetermined limit value. US Pat. No. 5,404,412 aims to suppress DC drift by using a buffer layer which is a transparent dielectric or insulator. This buffer layer is composed of silicon dioxide and II of the periodic table.
It is formed from a mixture of an oxide of at least one metal element selected from the group of metal elements of groups I to VIII, Ib and IIb and a semiconductor element other than silicon. For example, in the embodiment of US Pat. No. 5,404,412, 5 mol% In ₂ O ₃ and 5 mol% TiO 2.
_A buffer layer composed of silicon dioxide (SiO ₂ ) containing ₂ is disclosed. That is, the buffer layer is 5 m
containing ₂ % by weight of In ₂ O ₃ , the composition formula (Si
O _{2) 0.95} - _(and a TiO _{2) 0.05.} And it has been found that the DC drift of a waveguide device with such a buffer layer is within acceptable parameters.

[0009]

However, manufacturing a buffer layer having such a complicated composition is very complicated and the manufacturing cost is high. This is because, for example, the volatilization temperature and pressure of various material sources and the optimum film deposition conditions vary depending on the material.
Accurate control by the buffer layer) is extremely difficult.

Therefore, the present invention aims to at least alleviate some of the problems associated with the prior art described above.

[0011]

SUMMARY OF THE INVENTION Therefore, the first feature of the present invention is that the optical waveguide, the buffer layer, and the electrode formed on the buffer layer that influences the optical field in the optical waveguide in response to the applied electric field are And a buffer layer made of SiO _x , where x> 2, preferably x ≧ 2.2.

Thus, it has been found that by increasing the oxygen stoichiometry in SiO _x to the above mentioned ratio, the DC drift can be limited to within the permissible parameter range.

It should be noted that, when the above x exceeds 2, the DC drift velocity decreases to a level lower than that of the optical device having the conventional SiO ₂ buffer layer, particularly at x = 2.2. It has been found that it reaches a level comparable to the inherent drift of the substrate itself.

The performance of the optical device of the present invention is also advantageous by at least partially suppressing the rate of non-zero DC drift behavior in the optical device due to the imperfect electrical insulation of the device constituent materials. Be improved.

Preferably, the optical device is an electro-optical polarization (polarization) control device, an optical modulator such as a Mach-Zehnder optical intensity modulator, a variable optical attenuator, or an electro-optical switch.

A second aspect of the invention relates to a method of forming a buffer layer for an optical device, which method comprises vapor deposition, ion plating or sputtering of a silicon oxide film on a substrate in a relative vacuum atmosphere. And the step of flowing a mixed gas containing at least O ₂ gas onto the surface of the substrate and the step of ionizing the mixed gas by applying a bias voltage to the mixed gas are performed substantially at the same time. Is the way. The main purpose of the present production method is to adjust the flow rate of the mixed gas to achieve at least one of the desired partial pressure (partial pressure) and / or the applied bias voltage, so that the oxygen amount is x> 2, and preferably x ≧ 2.
Achieving a controlled chemical composition of the deposited SiO _x film to be 2.2.

The inventive form preferably provides a layer of silicon oxide that is substantially free of metal impurities, except for the elements that make up the substrate material.

Preferably, the present invention provides a layer formed so that the ratio of oxygen to silicon is x> 2 in SiO _x .

Preferably, in the inventive form, the buffer layer is composed of silicon and oxygen in a non-stoichiometric amount, ie in the super oxygen state. In a more preferred form of invention, the intentional doping of other elements, in particular metallic elements, is substantially not done.

The present invention uses a simple (compositional) silicon oxide as a buffer layer in order to solve the problems inherent in using a complicated buffer layer material such as metal-doped SiO ₂ , and The preferred Si / O ratio has the advantage of reducing DC drift.

A third aspect of the present invention relates to a method of forming a buffer layer in an optical device, which method comprises O ₂
Silica is deposited on the substrate in the presence of an activated or ionized mixed gas stream containing S and the buffer layer is S> 2.
It is a method including a step of being composed of an iO _x film.

[0022]

1 is a block diagram of a Mach-Zehnder modulator 1 according to the present invention.
00 shows a schematic sectional view. Waveguide arms 102, 104
Is formed in the electro-optic crystal substrate 108 of, for example, x-cut or y-cut lithium niobate by titanium diffusion. An input optical waveguide (not shown in FIG. 1) splits the received optical signal into waveguide arms 102 and 104.
The RF signal electrode 110 has two waveguide arms 102, 1
It will be placed between 04. The modulator 100 also includes external ground electrodes 112, 114 adapted to form a push-pull arrangement to modulate the phase of light propagating in the waveguide arms 102, 104. By recombining the phase-modulated optical signal via the output waveguide, an intensity-modulated optical signal is obtained from the output waveguide section (not shown). A signal electrode 110 and a ground electrode 112,
A buffer layer 116 is disposed below 114.

In the illustrated Mach-Zehnder modulator 100, the DC bias applying unit (not shown) for controlling the state of output intensity modulation has the same sectional structure as that of the illustrated sectional structure. The DC bias voltage is provided by the RF signal electrode 110. However, conventional x-cut and y
In a Mach-Zehnder modulator using cut lithium niobate, the buffer layer 116 is usually removed from the substrate surface, which causes a large DC drift of the output optical signal due to a significant reduction in the DC voltage applied through the buffer layer. I try to prevent it. This is to realize the electrodes in the x-cut and y-cut lithium niobate modulators by using the RF signal electrode together with the corresponding buffer layer and using the DC bias part without the DC buffer layer. is there. On the other hand, in the embodiment in which the z-cut lithium niobate is used as the substrate in the Mach-Zehnder modulator, the electrodes are always arranged so as to cover the waveguide so as to have the most effective push-pull arrangement. The buffer layer which has the function of reducing, preferably minimizing, the absorption of is not removed from the substrate surface, ie the buffer layer according to the invention is used to suppress DC drift.

To clearly show the effect of the present invention, several embodiments of Mach-Zehnder modulators using x-cut lithium niobate were tested. Specifically, the conventional x ≦
2 and Si of various stoichiometry including x> 2 of the present invention
A buffer layer of O _x was deposited on the surface of an x-cut lithium niobate substrate (wafer) with Ti-diffused waveguides. As a preferred embodiment of the Mach-Zehnder modulator, the RF electrode section and the DC bias electrode section are arranged in series on the same Mach-Zehnder waveguide structure as described above.
In addition, as a comparative example, a Mach-Zehnder modulator using a general x-cut lithium niobate substrate having an RF portion formed with a SiO ₂ buffer layer and a DC bias portion formed in a place without the buffer layer was also tested. I went to

The buffer layer in the embodiment of the present invention is
It was formed from SiO _{x in which} x is 2 or more. In a more preferred embodiment, this x is greater than or equal to 2.2.

Preferably, the buffer layer in embodiments of the present invention is manufactured using a silicon oxide film deposition apparatus located in a chemically reactive gas supply system such as an advanced plasma source (APS) E-beam system. Where Si
The bias voltage of the deposition system was varied to control the oxygen stoichiometry of the _Ox film. The bias voltage ionizes a portion of the feed gas, which flows towards the substrate during the deposition process.

In the preferred embodiment, the deposition apparatus is also used to vary the oxygen flow rate. Also, preferably,
By mixing an inert gas with oxygen during the manufacturing process of the embodiment,
The ionization state, that is, the degree of oxygen ionization and the degree of the reaction process of silicon and oxygen are changed.
Pure silica was used as the E-beam evaporation source.

Table 1 below shows main manufacturing conditions of the above-described embodiment according to the present invention.

[0029]

[Table 1]

FIG. 2 shows a graph 200 showing the change in oxygen stoichiometry as a function of the buffer layer thickness for the samples shown in Table 1. Here, the oxygen stoichiometry was determined by a secondary ion mass spectrometer (SIMS). The second sample (9211) 202 is the first sample (8772) 20.
Compared with No. 4, the change amount of the oxygen stoichiometry with respect to the layer thickness of the buffer layer is shown to be low. For sample 1, the x value of the oxygen stoichiometry is slightly greater than 2 (average about 2.07), while for sample 2 the x value is equal to 2.2, or in some cases 2. It was 2 or more.

FIGS. 3 and 4 show the DC drift performance measured at 80 ° C. as a function of time for the modulator of each embodiment having each SiO _x buffer layer formed under the conditions of Table 1. The graphs 300 and 400 are shown. Here, while applying an AC electric signal to the RF port of the modulator, the DC bias voltage applied to the DC bias port was controlled by the feedback control method, and the optical output intensity was traced at the preset state, that is, the quadrature modulation point. . It has been found that the embodiments exhibit a substantially linear DC drift behavior. The DC drift velocity was calculated for these embodiments.

Similar measurements and calculations of DC drift velocity were performed for other modulators of SiO _x where x was x <2. The oxygen stoichiometry of the buffer layer in other modulators is determined using a similar deposition system (APS) or other deposition systems such as plasma enhanced chemical vapor deposition (PECVD) and ion assisted deposition (IAD) systems. It was controlled by. For example, in the case of APS, the bias voltage is set to 130V and the O ₂ flow rate is set to 15
With the sccm set and the Ar flow rate set to 14 sccm, a SiO _x film having an x range of 1.74 ≦ x ≦ 1.82 was obtained.

FIG. 5 shows a graph 500 summarizing the relationship between measured DC drift velocity (logarithmic scale) and oxygen stoichiometry (measured by SIMS). In the comparative x-cut lithium niobate modulator without the buffer layer, its DC drift velocity is shown as "background" drift 502. It is observed that the DC drift decreases almost exponentially with increasing oxygen stoichiometry. Further, as is clear from FIG. 5, x ≧
One Embodiment 504 with 2.2 SiO _x Buffer Layer
The DC drift rate of the is approximately the same as the background drift rate (2 × 10 ⁻⁶ V / s), which is why the present invention addresses the DC drift problem of a lithium niobate modulator with a buffer layer. It turns out to be effective.

Although the above embodiment has been described based on the manufacture of the Mach-Zehnder optical modulator, the present invention is not limited to this. This embodiment can be realized even if the film of the SiO _x buffer layer is deposited on an integrated device and / or other device such as an electro-optical polarization (polarization) controller, an electro-optical switch, and a variable optical modulator. is there. Indeed, the embodiments of the present invention can be applied to any technical field requiring a stable DC dielectric layer.

The physical mechanism that enables the reduction of the DC drift exhibited by the embodiments of the present invention will be examined without being bound by any particular theory. The change in the ion concentration of Li and Nb with respect to the buffer layer thickness will be investigated by SIMS. According to the graph 600 in FIG. 6 showing the analysis result, Li
Is clearly present in the silicon oxide buffer deposited film as an impurity diffused from the lithium niobate substrate. It can be said that the effect of reducing the DC drift according to the embodiment of the present invention can be explained by suppressing the Li ⁺ ion migration in the silicon superoxide film. Chemical reaction bonding of excess oxygen in the silicon superoxide film can confine cation carriers such as Li ⁺ and H ⁺ , which substantially reduces migration between electrodes and preferably minimizes migration. I will suppress it to.

The present invention is equally applicable to various orientations of lithium niobate crystal substrates, ie, x-cut, y-cut, and z-cut substrates, as well as polarized amorphous materials and electro-optics. It is also applicable to other types of electro-optical substrates such as polymers and semiconductors.

Furthermore, the embodiment of the present invention is not limited to the manufacturing conditions shown in Table 1. Even if other combinations of manufacturing conditions are used, the optimum D
It is obvious to a person skilled in the art that SiO _x having C stability can be formed.

It should be noted that the reader of the specification of the present application will pay attention to all the papers and articles issued at the same time as or before the specification of the present application and published at the time of browsing the specification of the present application. The contents of papers and articles are incorporated by reference in the present specification.

All features disclosed in this specification, including the claims, abstract, and drawings, and / or all steps of the disclosed method or process are at least related to these features and / or steps. Any possible combination can be taken, except that the combinations are mutually exclusive.

Further, each feature disclosed in the present specification (including related claims, abstract and drawings) achieves the same or equivalent or similar object unless the content of the contrary is stated. It can also be replaced by a substitute feature. Thus, unless stated otherwise, each feature disclosed is only an example of a generic concept for equivalent or similar features.

The present invention is not limited to the contents of the detailed description of the above embodiment. The present invention can be broadly construed as being directed to any novel feature or combination of novel features among the features disclosed in this specification (including related claims, abstract and drawings), and The present invention can be broadly construed for any new step or combination of new steps among the plurality of steps in the methods and processes disclosed in the present specification.

[Brief description of drawings]

Embodiments of the present invention will be described by way of examples with reference to the following accompanying drawings.

FIG. 1 is a schematic cross-sectional view showing a Mach-Zehnder modulator.

FIG. 2 is a graph showing a change in oxygen stoichiometry with respect to a layer thickness of a buffer layer in the embodiment.

FIG. 3 Time (DC drift) in an embodiment with a SiO _x buffer layer where _x of SiO _x is about 2.04.
3 is a first graph showing changes in DC bias voltage with respect to FIG.

FIG. 4 Time (DC drift) in an embodiment with a SiO _x buffer layer where x of SiO _x is x ≧ 2.2.
3 is a second graph showing a change in DC bias voltage with respect to FIG.

FIG. 5 shows a log-linear relationship between DC drift velocity and oxygen stoichiometry of a buffer layer in various embodiments of the present invention and prior art devices.

FIG. 6 is a graph showing changes in impurities with respect to the layer thickness of the buffer layer in the embodiment.

Continued front page    (71) Applicant 502151820             1768 Automation Parkw             ay, San Jose, Califor             nia, USA, 95131 (72) Inventor Gary Gibson             Connecticut, United States 06070               Simsbury Binning Drive 1 F term (reference) 2H047 KA03 KB05 NA02 PA01 PA04                       PA05 QA03 QA04                 2H079 AA02 AA12 BA01 BA02 BA03                       CA05 CA08 DA22 EA05 EB05                       HA23

Claims (12)

[Claims] 1. A substrate is provided with an optical waveguide, a buffer layer, and at least one electrode formed on the buffer layer for influencing an optical field in the optical waveguide in response to an applied electric field. An optical device, characterized in that the layer is formed of SiO _x , said x being x> 2, preferably x ≧ 2.2. 2. The optical device according to claim 1, wherein the buffer layer is substantially not intentionally doped with metal ions. 3. The optical device according to claim 1, wherein the substrate is made of lithium niobate. 4. The optical device is an electro-optical polarization controller, and the optical device is any one of claims 1 to 3.
Optical device described in 3. 5. The optical device according to claim 1, wherein the optical device is an optical modulator including at least one Mach-Zehnder interferometer. 6. The optical device according to claim 1, wherein the optical device is an electro-optical switch. 7. The optical device according to any one of claims 1 to 3, wherein the optical device is a variable optical attenuator. 8. Forming a buffer layer in an optical device
Method, _TwoDeposit silica on substrate in the presence of flow
And set x to x> 2_xForm a film on the substrate
A method for forming a buffer layer, comprising the steps of: 9. The O ₂ stream is an active O ₂ stream or an ionized O ₂ stream.
9. The method of claim 8, wherein the method is _two- stream. 10. The method of claim 9, wherein the mixed gas is ionized by applying a bias voltage to the mixed gas. 11. The Si in which the buffer layer has x of x> 2, preferably x ≧ 2.2, by adjusting at least one of the flow rate of the mixed gas and the applied bias voltage.
The method according to claim 10, further comprising the step of forming O _x.
The method described. 12. The method according to claim 11, wherein the depositing step forms a silicon oxide film on the substrate by vapor deposition, ion plating, or sputtering deposition in a relative vacuum atmosphere.