Patent Application
20050068627
This application claims priority to Japanese Patent Application No. 2003-291165 filed Aug. 11, 2003 which is hereby expressly incorporated by reference herein in its entirety.
The present invention relates to a tunable optical filter that transmits light wavelength-selectively in order to extract a light component having a desired wavelength among a plurality of light components transmitted in an optical fiber and having different wavelengths in a wavelength division multiplexing (WDM) optical communication network and so on, and a method of manufacturing the same.
A conventional tunable optical filter utilizes the principle of a Fabry-Perot interferometer, and comprises a fixed mirror formed on a substrate and a movable mirror opposed to the fixed mirror in such a manner that an electrostatic gap is formed between the fixed and movable mirrors. In the tunable optical filter, drive voltage is applied between a movable electrode provided for the movable mirror and a fixed electrode provided for the fixed mirror so as to displace the movable mirror with respect to the fixed mirror, and thereby the length of the electrostatic gap can be varied. This electrostatic gap is formed by initially providing a sacrificial layer of given shape and size between the fixed mirror and movable mirror by utilizing a micro machining technique, and thereafter removing all or part of the sacrificial layer by etching (for example, refer to Japanese Unexamined Patent Publication No. 2002-174721 (claim 9, [0005], [0018], [0037], [0049]-[0056], and FIG. 6)). This art will be referred to as a first related art hereinafter.
In some conventional tunable optical filters, the electrostatic gap is formed using a silicon dioxide (SiO2) layer of an SOI (Silicon on Insulator) wafer as a sacrificial layer (for example, refer to U.S. Pat. No. 6,341,039 (Sixth-Seventh column, and FIGS. 4A-4I)). This art will be referred to as a second related art hereinafter.
In a tunable optical filter, drive voltage is applied to a parallel plate capacitor that is formed between a movable electrode provided for a movable mirror and a fixed electrode provided for a fixed mirror so as to generate electrostatic attraction between the movable and fixed mirrors, and thereby the movable mirror is displaced with respect to the fixed mirror. Here, in the case of applying drive voltage V to a parallel plate capacitor in which two pole plates of area S and distance d are opposed to each other with a dielectric of a dielectric constant ε therebetween, electrostatic attraction F, which acts on two pole plates, is represented by formula (1), as is well known.
F=({fraction (1/2)})·ε·(V/d)2·S (1)
In the first related art, the length of the electrostatic gap corresponding to the distance d is determined only based on the film thickness of a sacrificial layer. Even if a film-forming condition when manufacturing is set strictly, however, there may be a concern that a variation in the film thickness of sacrificial layers is caused. In the case where the variation is caused, even if the given drive voltage V is applied between the movable and drive electrodes, electrostatic attraction F that was expected by design for the drive voltage V can not be generated such that the movable mirror can not be displaced as designed. As a result, there has been a problem that, since the drive voltage for extracting a light component having each wavelength needs to be controlled and set for each tunable optical filter, the usability is not good. In addition, in the case where variation in the film thickness of the sacrificial layer is large, there may be a concern that a tunable optical filter that can not extract a light of a short-wavelength band or a light of a long-wavelength band among a plurality of light components transmitted in an optical fiber and having different wavelengths, is manufactured.
Meanwhile, in the second related art, since a movable mirror is not insulated from a drive electrode, there may be a case where, in the case where a large drive voltage is applied between a movable electrode and a drive electrode for any reason, a phenomenon referred to as sticking in which the movable mirror sticks to the drive electrode due to electrostatic attraction is caused and the movable mirror releases from the drive electrode even if the drive voltage is removed. In this case, the tunable optical filter can not be used from then on.
Furthermore, in either the first or second related arts, the sacrificial layer that has been formed is finally removed. In order to completely remove the sacrificial layer completely, usually, in a movable mirror, a movable electrode, and so on, a hole is formed on a top surface of the sacrificial layer, which is referred to as a release hole, for spreading an etchant that wet-etches the sacrificial layer across the entire area where the sacrificial layer is formed. Accordingly, since the area of the movable electrode decreases for the forming area of the release hole, the drive voltage V needs to be increased in order to generate a given electrostatic attraction F, as is apparent from the above formula (1), such that power consumption increases correspondingly. In addition, in either the first or second related arts, in the case where the length of the electrostatic gap is short, sticking attributed to the surface tension of water is caused when the sacrificial layer is removed. A tunable optical filter in which sticking is caused becomes a defective product.
The present invention is devised in order to solve such problems, and is intended to obtain a tunable optical filter whose electrostatic gap can be formed precisely, that can be driven with low drive voltage, and where sticking during manufacturing and while in use can be avoided, and a method of manufacturing the same.
In a tunable optical filter according to one aspect of the invention, a movable unit supporting a movable body that moves up and down freely and whose one surface has a movable mirror formed thereon, a drive electrode unit in which a drive electrode facing the movable body with a given electrostatic gap therebetween is formed, and an optical gap unit in which a fixed mirror facing the movable mirror with a given optical gap therebetween is formed, are joined to each other.
According to the invention, an electrostatic gap is formed precisely while a release hole is not formed in the movable body such that the tunable optical filter can be driven with low drive voltage.
In the tunable optical filter according to another aspect of the invention, an insulating film is formed on at least one of an area of the drive electrode that faces the movable body, and an area of the movable body that faces the drive electrode.
This enables sticking during manufacturing and while in use to be avoided.
In the tunable optical filter according to another aspect of the invention, an antireflection film formed on the other surface of the movable body is also used as the insulating film.
This enables the tunable optical filter to be constituted through less manufacturing processes at low cost.
In the tunable optical filter according to another aspect of the invention, the movable unit is composed of silicon. At least one of the drive electrode unit and the optical gap unit are composed of glass containing an alkali metal. At least one of the joining between the movable unit and the drive electrode unit, and the joining between the movable unit and the optical gap unit is implemented by anodic bonding.
According to the invention, an electrostatic gap is formed with extremely high precision. Accordingly, if a given drive voltage is applied between the movable body and the drive electrode, electrostatic attraction that was expected by design for the drive voltage can be generated such that the movable body can be displaced as designed. As a result, there is no need to control and set the drive voltage for extracting light components having each wavelength, for each tunable optical filter. Thus the usability is excellent, and all light components transmitted in an optical fiber that have different wavelengths can be extracted.
In a method of manufacturing a tunable optical filter according to another aspect of the invention, a first concave portion is formed in a first substrate, and then a drive electrode is formed on the first concave portion so as to form a drive electrode unit, in a first step. In addition, in a second step, a second concave portion is formed in a second substrate, and then a fixed mirror is formed on the second concave portion so as to form an optical gap unit. Next, in a third step, a third substrate on which an active layer having electrical conductivity, an insulating layer, and a base layer are sequentially deposited, is joined to the drive electrode unit in such a manner that the drive electrode faces the active layer, and then the base layer and the insulating layer are removed sequentially and a movable body is formed in the active layer, and thereafter a movable mirror is formed on the movable body. Then, in a fourth step, a structure that has been manufactured in the third step is joined to the optical gap unit in such a manner that the movable mirror faces the fixed mirror so as to manufacture the tunable optical filter.
According to the invention, a gap between the drive electrode and the movable body is formed without forming a sacrificial layer. Accordingly, a release hole for removing the sacrificial layer need not be formed in the movable body and so on such that the movable body having the area as designed can be obtained. Thus a manufactured tunable optical filter can be driven with a low drive voltage such that power consumption can be reduced.
In a method of manufacturing a tunable optical filter according to another aspect of the invention, a first concave portion is formed in a first substrate, and then a drive electrode is formed on the first concave portion so as to form a drive electrode unit, in a first step. In addition, in a second step, a second concave portion is formed in a second substrate, and then a fixed mirror is formed on the second concave portion so as to form an optical gap unit. Next, in a third step, a third substrate on which an active layer having electrical conductivity on which a movable mirror is formed, an insulating layer, and a base layer are sequentially deposited, is joined to the optical gap unit in such a manner that the movable mirror faces the fixed mirror, and then the base layer and the insulating layer are removed sequentially and a movable body is formed in the active layer. Then, in a fourth step, a structure that has been manufactured in the third step is joined to the drive electrode unit in such a manner that the movable body faces the drive electrode.
According to the invention, a gap between the drive electrode and the movable body is formed without forming a sacrificial layer. Accordingly, a release hole for removing the sacrificial layer need not be formed in the movable body and so on such that the movable body having the area as designed can be obtained. Thus a manufactured tunable optical filter can be driven with a low drive voltage such that power consumption can be reduced.
In the method of manufacturing a tunable optical filter according to another aspect of the invention, an insulating film is formed on an area to face the movable body of the drive electrode, in the first step.
Furthermore, in the method of manufacturing a tunable optical filter according to another aspect of the invention, the joining is implemented in such a manner that the drive electrode faces the active layer after an insulating film is formed on an area to face the drive electrode as the movable body of the active layer, in the third step.
Moreover, in the method of manufacturing a tunable optical filter according to another aspect of the invention, an insulating film is formed on an area to become the movable body and face the drive electrode before the movable body is formed, in the third step.
According to the invention, sticking during manufacturing and while in use can be avoided.
In the method of manufacturing a tunable optical filter according to another aspect of the invention, the insulating film and an antireflection film are formed on an area to become the movable body of the active layer, in the third step.
Furthermore, in the method of manufacturing a tunable optical filter according to another aspect of the invention, the insulating film and an antireflection film are formed on an area to become the movable body before the movable body is formed, in the third step.
According to the invention, sticking during manufacturing and while in use can be avoided while a tunable optical filter can be constituted through less manufacturing processes at low cost.
In the method of manufacturing a tunable optical filter according to another aspect of the invention, the active layer is composed of silicon. At least one of the first substrate and the second substrate are composed of glass containing an alkali metal. The joining is implemented by anodic bonding in at least one of the third step and the fourth step.
According to the invention, an electrostatic gap is formed with extremely high precision. Accordingly, if a certain drive voltage is applied between the movable body and the drive electrode, electrostatic attraction that was expected by design for the drive voltage can be generated such that the movable body can be displaced as designed. As a result, there is no need to control and set the drive voltage for extracting light components having each wavelength, for each tunable optical filter. Thus the usability is excellent, and all light components transmitted in an optical fiber that have different wavelengths can be extracted.
FIGS. 3(1)-3(6) are diagrams showing a manufacturing process of the tunable optical filter.
FIGS. 4(1)-4(2) are diagrams showing a manufacturing process of the tunable optical filter.
FIGS. 5(1)-5(2) are diagrams showing a manufacturing process of the tunable optical filter.
FIGS. 7(1)-7(4) are diagrams showing a manufacturing process of the tunable optical filter.
FIGS. 8(1)-8(6) are diagrams showing a manufacturing process of the tunable optical filter.
The tunable optical filter of the embodiment comprises a drive electrode unit 1, a movable unit 2, and an optical gap unit 3. An electrostatic gap EG whose length is about 4 micrometers is formed between the drive electrode unit 1 and the movable unit 2. An optical gap OG whose length is about 30 micrometers is formed between the movable unit 2 and the optical gap unit 3. The drive electrode unit 1 is constituted by forming a drive electrode 12 and an insulating film 13 that have a substantially ring shape, on a concave portion 11a formed in a substantially center part of a glass substrate 11 whose section has a substantially U-shape. The glass substrate 11 is composed of glass containing alkali metal such as sodium (Na) and potassium (K) for example. As glass of this kind, for example, borosilicate glass containing an alkali metal, specifically, Pyrex (registered trademark)•glass from Corning Co. is named. In the case of joining the drive electrode unit 1 to the movable unit 2 by anodic bonding (to be described later), glass constituting the glass substrate 11 is required to have almost same coefficient of thermal expansion as that of silicon constituting the movable unit 2 since the glass substrate 11 is heated. Thus, among the Pyrex (registered trademark)•glass, Corning #7740 (brand name) is preferable.
The drive electrode 12 is composed of metal such as gold (Au) and chromium (Cr), or a transparent conductive material for example. As transparent conductive materials, for example, there are tin oxide (SnO2), indium oxide (In2O3), and indium tin oxide (ITO). The film thickness of the drive electrode 12 is 0.1-0.2 micrometers for example. The drive electrode 12 is coupled to a terminal provided outside the glass substrate 11 with wiring therebetween, although not shown in the drawing. The insulating film 13 is composed of silicon dioxide (SiO2) or silicon nitride (SiNx) for example, and is formed in order to prevent sticking between the drive electrode 12 and a movable body 21a to be described later.
The movable unit 2 comprises a movable unit substrate 21, an antireflection film 22, and a highly reflective film 23. The movable unit substrate 21 is composed of silicon dioxide (SiO2) for example, and has a film thickness of about 4 micrometers. As shown in
The antireflection film 22 is formed on almost the entire area of lower surface of the movable body 21a, and is formed of a multi-layered film in which silicon dioxide (SiO2) thin films and tantalum pentoxide (Ta2O5) thin films are deposited alternately. The antireflection film 22 prevents light incident from below at generally the center of the drive electrode unit 1 (refer to the arrowhead of
The optical gap unit 3 comprises the glass substrate 31, the highly reflective film 32, and an antireflection film 33. The glass substrate 31 is composed of glass whose material is same as that of the glass substrate 11, and the section thereof has a substantially doubly-supported-beam shape in which a concave portion 31a is formed in a substantially center part thereof. The highly reflective film 32 is formed on a lower surface of the concave portion 31a of the optical gap unit 3, into a substantially disc shape, and is formed of a multi-layered film in which silicon dioxide (SiO2) thin films and tantalum pentoxide (Ta2O5) thin films are deposited alternately. The highly reflective film 32 is an element for reflecting light incident from below at substantially the center of the movable unit 2 in
Next, a method of manufacturing a tunable optical filter having the above structure will be described referring to
Next, the entire top surface of the metal film 15 is coated with photo resist (not shown in the drawing) and then the photo resist applied to the entire top surface of the metal film 15 is exposed using a mask aligner. Thereafter, by using a photolithography technique, in which developing is implemented using a developer, a photo resist pattern (not shown in the drawing) is formed in order to later form a portion to become the concave portion 11a (refer to
Next, by using a wet-etching technique, an unnecessary portion of the glass substrate 14 is removed with a hydrofluoric acid (HF) for example, so as to form the concave portion 11a shown in
Next, in order to fabricate the movable unit 2, an SOI substrate 24 shown in
Next, the drive electrode unit 1 shown in
Next, the base layer 25 is removed from the structure shown in
(1) Wet-Etching Removing Method
By immersing the structure shown in
Si+2KOH+H2O→K2SiO3+2H2 (2)
In this case, since the etching rate of silicon (Si) is much larger than that of silicon dioxide (SiO2), the insulating layer 26 composed of silicon dioxide (SiO2) functions as an etchant stopper for the active layer 27 composed of silicon (Si).
As etchants used in this case, other than the above water solution of potassium hydroxide (KOH), a water solution of tetramethyl ammonium hydroxide (TMAH), which is widely used as a semiconductor surface treating agent and a developer for positive resist for photolithography, a water solution of ethylenediamine pyrocatechol diazine (EPD), a water solution of hydrazine, and so on, are listed.
Using this wet-etching removing method enables batch treatment in which a group of the structures shown in
(2) Dry-Etching Removing Method
The structure shown in
2XeF2+Si→2Xe+SiF4 (3)
In this case, since the etching rate of silicon (Si) is much larger than that of silicon dioxide (SiO2), the insulating layer 26 composed of silicon dioxide (SiO2) functions as an etchant stopper for the active layer 27 composed of silicon (Si). Since the dry-etching in this case is not plasma-etching, the glass substrate 11 and the insulating layer 26 are less likely to be damaged. Other than dry-etching using the xenon difluoride (XeF2), there is plasma-etching using carbon tetrafluoride (CF4) or sulfur hexafluoride (SF6), for example,.
Next, by using a wet-etching technique, for the structure shown in
Next, with respect to the structure for which the anisotropic etching has been implemented, a photo resist pattern (not illustrated) is removed using oxygen plasma for example, so as to obtain the movable unit substrate 21 as shown in
Silicon dioxide (SiO2) thin films and tantalum pentoxide (Ta2O5) thin films, of about 10-20 layers for example, are deposited alternately using a CVD device and a PVD device, on the substantially center part of top surface of the movable unit substrate 21, and thereby the highly reflective film 23 shown in
Then, in order to fabricate the optical gap unit 3, on a top surface of a glass substrate 34 (refer to
Next, the entire top surface of the metal film 35 is coated with photo resist (not shown in the drawing), and then a photo resist pattern (not shown in the drawing) is formed by using the photolithography technique in order to later form a portion to become the concave portion 31a (refer to
Next, by using a wet-etching technique, an unnecessary portion of the glass substrate 34 is removed with a hydrofluoric acid (HF) for example, so as to form the concave portion 31a shown in
Next, the structure shown in
Next, the operation of a tunable optical filter having the above structure will be described referring to
A plurality (for example 60-100) of light components having infrared wavelengths enters the tunable optical filter from below at substantially the center of the drive electrode unit 1 (refer to an arrowhead of
In the process in which light components are repeatedly reflected between the highly reflective film 32 (fixed mirror) and the highly reflected film 23 (movable mirror), light whose wavelength does not satisfy interference condition corresponding to the distance between the highly reflective film 32 and the highly reflective film 23 (optical gap OG) is abruptly attenuated, while only light whose wavelength satisfies this interference condition is left so as to be finally emitted from the tunable optical filter. This is the principle of a Fabry-Perot interferometer. Since light whose wavelength satisfies this interference condition is transmitted, the wavelength of a light to be transmitted can be selected if the movable body 21a is displaced and the optical gap OG is changed by changing drive voltage.
As described, the tunable optical filter according to the embodiment comprises the drive electrode unit 1 having the glass substrate 11, the movable body 2 composed of silicon (Si), and the optical gap unit 3 having the glass substrate 31, such that the electrostatic gap EG is formed precisely. In the case of using anodic bonding especially, the electrostatic gap EG is formed with extremely high precision. Accordingly, if a given drive voltage is applied between the movable body 21a and the drive electrode 12, electrostatic attraction that was expected by design for the drive voltage can be generated such that the movable body 21a can be displaced as designed. As a result, there is no need to control and set the drive voltage for extracting light having each wavelength, for each tunable optical filter. Thus the usability is excellent, and all light transmitted in an optical fiber having different wavelengths can be extracted.
Furthermore, in the tunable optical filter according to the embodiment, the electrostatic gap EG is formed without forming a sacrificial layer, while the insulating film 13 is formed on the drive electrode 12. Thus, even if the length of the electrostatic gap EG is set to be short, unlike in the case of the first and second related arts, sticking can be prevented both during manufacturing and while in use. As a result, process yield and endurance can be improved. In addition, in the tunable optical filter of the embodiment, a sacrificial layer is not formed in the manufacturing process. Thus there is no need to form a release hole for removing the sacrificial layer in the movable unit substrate 21 and so on such that the movable body 21a having the area as designed can be obtained. Accordingly, compared to the first and second related arts, the tunable optical filter can be driven with lower drive voltage such that power consumption can be reduced.
Moreover, in the tunable optical filter of the embodiment, since the concave portion 31a is formed by implementing glass etching of high precision for the glass substrate 34, and the optical gap unit 3 is joined to the movable unit 2, especially by anodic bonding, the optical gap OG is also formed precisely. The tunable optical filter therefore can be stably driven. In addition, in the tunable optical filter of the embodiment, the glass substrate 31, which is transparent, also serves as a sealing cap such that the operation of the tunable optical filter can be monitored.
Furthermore, in the tunable optical filter of the embodiment, since the movable body 2 is formed from the SOI substrate 24, the movable body 21a having a precise film thickness can be formed. In the case of using a commercially available substrate as the SOI substrate 24, since a surface of the active layer 27 has been already mirror-finished by the manufacturer, utilizing this enables the antireflection film 22 and the highly reflective film 23 of high precision to be formed.
Although the embodiment has been described referring to drawings above, the particular structure is not limited to the embodiment. Modification of design and so on without departing from the scope and spirit of the present invention is also included in the present invention.
For example, although the example in which the SOI substrate 24 is used to fabricate the movable unit 2 has been illustrated in the embodiment, the invention is not limited to this. Others may be used. For example, ann SOS (Silicon on Sapphire) substrate may be used. Otherwise, a substrate formed by attaching a top surface of a silicon substrate whose top surface has a silicon dioxide (SiO2) film formed thereon and a top surface of other silicon substrate, may be used.
In addition, although the example in which both of the drive electrode unit 1 and the optical gap unit 3 are formed of a glass substrate has been illustrated in the embodiment, the invention is not limited to this. The drive electrode unit 1 and the optical gap unit 3 may be composed of, for example, materials through which a light of desired transmission wavelength band such as infrared, such as silicon, sapphire, and germanium for example.
Although the example in which the number of the hinges 21b is 4 (four) has been illustrated in the embodiment, the invention is not limited to this. The number of the hinges may be, for example, 3 (three), 5 (five), 6 (six), or more. In this case, the hinges are formed on the periphery of the movable body 21a so that the distance between the adjacent hinges is equal to each other. In addition, although the example in which the movable unit 2 is formed after the drive electrode unit 1 is joined to the structure shown in
Although the example in which the insulating film 13 is formed on the drive electrode 12 has been illustrated in the embodiment, the invention is not limited to this. An insulating film may be formed on an area that is a lower surface of the movable body 21a and faces at least the drive electrode 12. As a method of forming this insulating layer, by using thermal oxidization in which silicon is heated under oxidizing atmosphere, and a TEOS (Tetra Ethyl Ortho Silicate)-CVD device for example, a silicon dioxide (SiO2) film is formed. Meanwhile, both of the silicon dioxide (SiO2) film and the tantalum pentoxide (Ta2O5) film that constitute the antireflection film 22 formed on an under surface at substantially the center of the movable body 21a, are also an insulator. The antireflection film 22 therefore may be formed on the entire lower surface of the movable body 21a so as to be also used as the insulating film. In this case, with respect to peripheral part of a lower surface of the movable body 21a, there is no need to form a number of layers that would be sufficient to function as the antireflection film 22. Only layers in a number sufficient to function as an insulating film may be formed. Moreover, both the insulating film 13 and an insulating film formed on a lower surface of the movable body 21a may be formed. As described, if the antireflection film 22 is also used as an insulating film, the same advantageous effect as that of the above embodiment can be obtained through less manufacturing processes such that a tunable optical filter can be made at low cost. Furthermore, although the example in which the highly reflective film 32 is formed on the entire lower surface of the optical gap unit 3 has been illustrated in the embodiment, the invention is not limited to this. The highly reflective film 32 may be formed only on an area that faces the highly reflective film 23 of the lower surface of the optical gap unit 3.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-291165 | Aug 2003 | JP | national |
