Abstract:
The present innovation discloses the design, configuration and process of a kind of MEMS-based tunable optical devices. The devices can be classed as dual-cavity of Fabry-Perot resonators, which consist of a first outer membrane, a middle membrane and a second outer membrane. And these membranes are separated by cavities. The membranes will deflect under electrostatic force and the thicknesses of cavities will change. The numbers of the layers of membranes should satisfy an equation. These devices can be adopted for optical switches, VOA or tunable filters.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This Application claims a priority date of Nov. 3, 2003 benefited from a previously filed Provisional Patent Application 60/517,021 filed on Nov. 3, 2003 by the Applicants of this Formal Patent Application 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not Applicable 
   REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
   Not Applicable 
   BACKGROUND OF THE INVENTION 
   The present innovation is related to the optical telecommunication, Particularly to the design and manufacture of tunable optical devices for WDM (Wavelength Division Multiplexed) systems. WDM technology highly expends the capability of modem optical communication network. Tens of wavelengths located between 1530 to 1570 mn can pass through a signal fiber by this technology. Each wavelength corresponds to a Channel. With the development of the optical telecommunication, the network becomes more and more complicated in structures. The network should be intelligent, meaning be more flexible and scalable. These characteristics strongly depend on tunable devices, such as optical switches, variable optical attenuators (VOA) and tunable filters. 
   Optical switches are used to selectively reroute signals or to control the traveling direction of light beams. VOA are used to adjust light intensities so the network could work in order. Tunable filters are used to select a certain wavelength among many wavelengths and the network can be dynamically managed. 
   These devices are basic in optical network, and they should have the characteristics such as small size, low power consumption, high reliability, high tuning speed and small insertion loss. Until now, switches, VOA and tunable filters can be made based on different principles, such as mechanical moving, magneto optical effect, electro optical effect, acoustic optical effect and thermal effect. For devices based on magneto optical effect, the power consumption is high. And due to the magnetic parts, they are hard to be made small. For devices based on electro optical effect, the applied voltage is usually quite high. For devices based on acoustic optical effect, side lobes are strong and insertion loss is large. For devices based on thermal effect, the tunable range is small and the response time is usually long. 
   As for the mechanical principle, it can be divided in several methods. Some devices adopt motors to drive optical parts, some adopt piezocrystal. All these methods need heavy labor on assembly. Recently, MEMS technology has been introduced into manufacturing optical devices. It adopts the state-of-the-art technology of semiconductor to batch fabricate small-sized devices. 
   For MEMS technology, reflection by micro-mirrors can be adopt to make VOA and switches, while the robust of micro-mirrors is poor; Diffraction by grating is only good for making VOA; Interference devices based on the Fabry-Perot cavity can have very narrow resonant frequency, and is okay to be used for monitoring channels. However, it is poor in re-routing channels. Every optical channel has bandwidth due to modulation, and some information of channels will be filtered out and the error rate will increase if we use the Fabry-Perot cavity as tunable filters to select channels. This shortcoming will become severe in DWDM. Interference devices based on the Fabry-Perot cavity have poor performance for VOA and switches since the wavelength dependent loss (WDL) is high. 
   Fabry-Perot resonators with multiple-cavity can have flat passband with high resolution. Tunable filters having this structure will be suitable for re-routing channels. When the reflectivity of the resonators is low, this structure can also be adopted for VOA and Switches. Since this structure requires substantially identity of cavities, Macro devices based on this structure is difficult to be made and lack mechanical stability. U.S. Pat. No. 6,424,466 give a related MEMS-based design of tunable filters, but it is mainly on the process of assembling. 
   BRIEF SUMMARY OF THE INVENTION 
   The present innovation discloses the structure and process of a kind of MEMS-based tunable optical devices. These devices can be adopted for optical switches, VOA or tunable filters. These tunable devices have some advantages. First, by using MEMS technology, the size of these tunable devices can be very small, and these devices can be batch fabricated. Second, the mechanical robustness is improved, for moving is achieved by deflection of membranes which have less mass. Third, the power consumption is almost neglectable, for these optical devices are driven by electrostatic force. Forth, no more optical coating is needed, and all optical functions are achieved by membranes themselves. 
   In the present innovation, the devices for optical switches, VOA and tunable filters have the same optical structures. They all can be classed as dual-cavity of Fabry-Perot resonators, which consist of a first outer membrane, a middle membrane and a second outer membrane. And these membranes are separated by a first cavity and second cavity, respectively. The membranes comprise alternatively layers of high refractive index materials and low refractive index materials. The two outer membranes have lower reflectivity than the middle one does. And the number of the total layers of each membrane is odd. To increase the reflectivity, the first layer of each membrane is high refractive index layer. Since the number of the total layer is odder, the last layer of each membrane is also high refractive index layer. Another advantage of odd layers is that the membranes are symmetric to the middle layers and the stresses are balanced. The first and second outer membrane can comprise at least one high refractive index layers. In the present innovation, the high refractive index material is polysilicon or amorphous silicon, and the low refractive material is silicon nitride. They are standard materials in semiconductor industry. The optical thickness of every layer is the odder multiplex time of a quarter of light wavelength, preferably 1 or 3 times of a quarter wavelength are chosen. 
   Let N 1  denote the number of the layers of the first outer membrane, N 2  denote the number of the layers of the second outer membrane and N 3  denote the number of the layers of middle membrane. The present innovation discloses an equation of the relationship of these numbers: N 1 +N 2 +5=N 3 . When they satisfy the equation, the devices will have a flat passband with few ripples. 
   The present innovation also discloses the process of making such devices. The state-of-the-art semiconductor technology will insure the thicknesses of membranes and cavities, and no manual assembly is needed on chip level. 
   In the present innovation, membranes are deposited on substrates, and the substrate is preferably doped silicon. The membranes are electrical conductive and will deflect under electrostatic forces between membranes or between membranes and substrates, causing the change of the thicknesses of cavities. And optical properties of the devices will vary consequently. 
   The embodiments of the present invention can comprise one substrate or two substrates. All the materials for membranes and cavities are deposited on the substrates. The embodiments having two substrates are formed by bonding or gluing the two substrates. 
   The out membranes of the embodiment adopted for tunable filters have hat-top structure which will benefit the optical properties of device. The surfaces of the hat-top structures will become curved when the membranes are released. 
   The embodiment having one substrate can be used for switches and VOA, and the embodiment having two substrates can be used for switches, VOA and tunable filters. 
   More features and advantages of the present innovation will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the present invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the present invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings are not necessarily to scale; emphasis has instead been placed upon illustrating the principles of the invention. Of the drawings: 
       FIG. 1  illustrates the sequence of deposition of membranes and silicon dioxide layers on a substrate. 
       FIG. 2  illustrates some holes are etched through the membranes and dioxide layers around light path. 
       FIG. 3  illustrate a hole is etched through on a substrate. 
       FIG. 4  illustrates the structure of a device having one substrate. 
       FIG. 5  shows a schematic configuration of VOA containing the device having one substrate. 
       FIG. 6  shows a schematic configuration of switches containing the device having one substrate. 
       FIG. 7  shows an alternative schematic configuration of switches with a circulator. 
       FIG. 8  illustrates the sequence of deposition on two substrates, and the outer membranes have detailed structures. 
       FIG. 9  illustrates the deposition of a middle membrane on a polished surface. 
       FIG. 10  illustrates the bonding or gluing of the two substrates face to face. 
       FIG. 11  illustrates the structure of a device having two substrates. 
       FIG. 12  illustrates a recess made on the surface of a second substrate before the deposition of membranes and dioxide layers. 
       FIG. 13  illustrates a middle membrane is deposited after polish of dioxide layer only on the first substrate. 
       FIG. 14  illustrates some cavities forming on the two substrates by wet etching. 
       FIG. 15  illustrates an alternative embodiment of the device having two substrates. 
       FIG. 16  shows a schematic configuration of tunable filters containing the device having two substrates. 
       FIG. 17  shows a schematic configuration of add/drop complexers containing the device having two substrates. 
       FIG. 18  shows a alternative schematic configuration of add/drop multiplexers. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic cross-sectional view illustrating the process for fabricating a device having one substrate according to the present invention. At first, a layer of silicon dioxide  120  is deposed on a substrate  111 . This silicon dioxide can be LTO, PSG, BPSG or phosphor doped LTO. When it mentioned below, silicon dioxide in the present innovation means one of the following materials: LTO, PSG, BPSG and phosphorus doped LTO. Substrate  111  is electrically conductive. The thickness of silicon dioxide layer  120  is in the range of 2 to 5 micrometer. Then a first outer membrane  130  is deposited on the dioxide layer  120 . A first outer membrane  130  is formed by alternatively deposition of the layers of high refractive index materials and low refractive index materials with LPCVD or PECVD. The optical thicknesses of these layers are the odder multiplex time of quarter of light wavelength. The high refractive index material is polysilicon or amorphous silicon, and the low refractive index material is nitride. A part of the dioxide layer  120  will be etched off to release the membrane  130 , and the other parts support the membrane  130 . 
   After the first outer membrane  130  is formed, a silicon dioxide layer  140  is deposited, followed by the alternative deposition of high and low refractive index materials to form a middle membrane  150 . And on the middle membrane  150 , a layer of silicon dioxide  160  is deposited. On the dioxide layer  160  a second outer membrane  170  is deposited. 
   The structures and deposition process of the first outer membrane  130 , the middle membrane  150  and the second outer membrane  170  are the same, which are formed by alternative layers of polysilcion/amorphous silicon and nitride. And the optical thicknesses of these layers are the odder multiplex time of quarter of light wavelength. The first layer and the last layer of these membranes are polyisilicon or amorphous layers. The membrane  130 , 150  and  170  are multi-layer dielectric membranes. In the present innovation, said multi-layer dielectric membranes are composed by alternative layers of high refractive index material an low refractive index material. In the present innovation, the high refractive index material is polysilicon or amorphous silicon, and the low refractive material is nitride. 
   Generally, N 1 , the number of the total layers of the first outer membrane  130 , and N 2 , the number of the total layers of the second outer membrane  170 , are the same. And N 3 , the number of the total layers of the middle membrane  150 , satisfies the equation: N 1 +N 2 +5=N 3 . 
     FIG. 2  illustrates, around light path of the device, some holes  180  are made through the membrane  170 , dioxide layers  160 , membrane  150 , dioxide layer  140  and membrane  130 . Photo resist can be used as mask and the holes  180  can be made by a dry etcher. 
     FIG. 3  shows a hole  190  is etched through on the substrate  111 . Light beam can shine on outer membranes directly. Then the silicon dioxide layers on substrate  111  are etched by HF. After certain time, the devices are cleaned and fully dried.  FIG. 4  illustrates the final structures of the device  110 . Around the holes  180  and  190 , a part of dioxide layer  120 ,  140  and  160  are etched off. And a non-optical cavity  121 , a first optical cavity  141  and a second optical cavity  161  are formed. 
   The first layers or the last layers of the first outer membrane  130  and second outer membrane  170  are doped so they are electrically conductive. In the light path, doping is optional to decrease the absorption of light. In the regions corresponding to the peripheries of chip dies, the first and the last layer are not doped to decrease the possibility of current leakage. The middle membrane  150  can be doped or not. 
   The stresses of these membranes are tensile. This can be achieved by controlling the parameters of deposition of these membranes. 
   Two adjustable voltages are applied to generate electrostatic forces to vary the optical properties of the devices  110 . In the case that the middle membrane  150  is not doped, there are two methods to apply voltages on the device  110 . 
   One method is as following: an adjustable voltage is applied between the substrate  111  and the first outer membrane  130 , another adjustable voltage is applied between the first outer membrane  130  and the second outer membrane  170 . When the voltage between the first and second outer membrane increases, the first and second outer membrane will attract each others and the first cavity  141  and second cavity  161  will become thinner. The voltage between the substrate  111  and first outer membrane  130  is used to modify the thickness of the first cavity  141  to insure the thickness identity of both cavities. The middle membrane bears no electrostatic force and it does not move. This method makes the first cavity  141  and second cavity  161  thinner. The original thicknesses of the first cavity  141  and second cavity  161  can be one wavelength. When no voltages applied, the light incident will fully pass through the devices. When voltages increased, the first cavity  141  and second cavity  161  can become three quarter of wavelength, and the light incident is fully reflected. The original thicknesses of the first cavity  141  and second cavity  161  can also be three quarter of wavelength. 
   The other method is like this: an adjustable voltage is applied between the substrate  111  and the first outer membrane  130 , another adjustable voltage is applied between the first outer membrane  130  and the second outer membrane  170 . When these two voltage increase, due to the electrostatic force, the substrate  111  attracts first outer membrane  130 , at mean time the first outer membrane  130  and the second outer membrane  170  attract each others. The first outer membrane  130  bears more electrostatic force to the substrate  111  and it moves to the substrate  111 . The second outer membrane  170  also moves to the substrate  111  due to the force between membrane  130  and  170 . Since the middle membrane  150  does not move, the first cavity  141  becomes thicker and the second cavity  161  becomes thinner. The original thickness of the first cavity  141  and second cavity  161  can be half of the wavelength. When no voltage applied, the light incident will fully passed the device  110 . When voltage applied and the first cavity  141  become three quarter of wavelength and second cavity  161  decreases to one quarter of wavelength, the light incident will be fully reflected. Also the original thicknesses of the first cavity  141  and second cavity  161  can be one of the following sets of values, respectively: three quarter wavelength and three quarter wavelength, one quarter wavelength and three quarter wavelength, half wavelength and one wavelength. 
   In the case that the middle membrane  150  is doped, one adjustable voltage is applied between the substrate  111  and the first outer membrane  130  and another adjustable voltage is applied between the middle membrane  150  and the second outer membrane  170 . When these voltages increase, the first outer membrane  130  is attracted to the substrate  111  and the first cavity  141  becomes thicker. At meantime the middle membrane  150  and the second outer membrane  170  attract each others, and the second cavity  161  becomes thinner. The first outer membrane  130  and the middle membrane  150  are connected to common electrode (ground) so the first and second cavity can be tuned independently. The original thickness of the first cavity  141  and second cavity  161  can be one of the following sets of values, respectively: half wavelength and one wavelength, three quarter wavelength and three quarter wavelength, one quarter wavelength and three quarter wavelength, half wavelength and half wavelength. 
     FIG. 5  shows a scheme of VOA adopting the device  110 . Light from a fiber  601  is focused by a lens  602  and shine on the device  110 . On the other side, a lens  603  collects the passing through light to a fiber  604   
     FIG. 6  shows a scheme of switches adopting the device  110 . Light from a fiber  605  is focused by a lens  607 , on certain conditions, light can be fully pass through the device  110 . A lens  608  collects the light, and a fiber  609  output the light. If the first and second cavities are changed, light can be fully reflected back. The lens  607  will collect the reflected light and a fiber  606  will output the light. 
     FIG. 7  shows another scheme of switcher using a circulator  610 . 
   According to the present innovation, devices with two substrates can also be made.  FIG. 11  shows the one embodiment for the device having two substrates, and  FIG. 8  to  FIG. 10  show the process to make such devices. 
   In  FIG. 8 , layers of silicon dioxide  220 - 1  and  220 - 2  are deposited on the first substrate  211 - 1  and the second substrate  211 - 2 , respectively. Then the first outer membrane  230 - 1  and the second outer membrane  230 - 2  are deposited on the dioxide layer  220 - 1  and  220 - 2 , respectively. The first outer membrane  230 - 1  and the second outer membrane  230 - 2  are multi-layer dielectric membranes, having the same structure as the first outer membrane  130  in  FIG. 1 . And the numbers of the total deposited layers of the first outer membrane  230 - 1  and second outer membrane  230 - 2  are N 4  and N 5 , respectively. The most part of the membranes  230 - 1  and  230 - 2  are thinned by etching, leaving the hat-top structure  231 - 1  and  231 - 2  on light path. This can be done by using photo resist as protecting mask. And the holes  233 - 1  and  233 - 2  can be made by dry etching around the hat-top structure  231 - 1  and  231 - 2 , respectively. Then the silicon dioxide layer  240 - 1  and  240 - 2  are deposited over membranes  230 - 1  and  230 - 2 , respectively. The dioxide layer  240 - 1  and  240 - 2  are not flat due to the holes and hat-top structures of the first outer membrane  230 - 1  and second outer membrane  230 - 2 , and they are polished to have flat and smooth surfaces. After polishing, the thicknesses of dioxide layer  240 - 1  and  240 - 2  are the same, and in the range of 6 to 30 micrometer. 
     FIG. 9  shows the middle membrane  250  is deposited on the dioxide layer  240 - 1 . The middle membrane  250  is a multi-layer dielectric membrane, and the deposition method and structure of the middle layer  250  is the same as the middle layers  150  in  FIG. 1 . An optional step here is that some holes around light path can be etched through the middle membrane  250 . 
   Let N 6  denote the number of the total layers of the middle membrane  250 . N 4 , N 5  and N 6  satisfy the equation: N 4 +N 5 +5=N 6 . 
     FIG. 10  shows the first substrate  211 - 1  and second substrate  211 - 2  are bonded or glued face to face together, forming a device  210 . And on light path the holes  260 - 1  and  260 - 2  are etched through on the first substrate  211 - 1  and the second substrate  211 - 2 , respectively. 
     FIG. 11  illustrates the forming of the first non-optical cavity  221 - 1 , the first cavity  241 - 1 , the second cavity  241 - 2  and the second non-optical cavity  221 - 2  after the device  210  is etched by HF for certain time and fully cleaned and dried. Due to the gradient of tensile stress of the first outer membrane  230 - 1  and second outer membrane  230 - 2 , structure  231 - 1  and  231 - 2  will have a curved surface after releasing. The curved surfaces benefit the optical property of tunable filters. 
     FIG. 12  to  FIG. 15  illustrate an alternative process and embodiment for the two-substrate devices. In  FIG. 12 , a recess  315  is made at the surface of the second substrate  311 - 2 , and the depth of the recess  315  is in the range of 6 to 30 micrometer. Then on the first substrate  311 - 1  and second substrate  311 - 2 , the silicon dioxide layers  320 - 1  and  320 - 2  with thickness of 2 to 5 micrometer are deposited, respectively. Over the dioxide layer  320 - 1  and  320 - 2 , the first outer membrane  330 - 1  and second outer membranes  330 - 2  are deposited. The First outer membrane  330 - 1  and second outer membrane  330 - 2  have the same structure as the first outer membrane  230 - 1  in  FIG. 8 . The most part of the membranes  330 - 1  and  330 - 2  are thinned by etching, leaving a hat-top structure  331 - 1  and  331 - 2  on light path. This can be done by using photo resist as protect mask. And holes  333 - 1  and  333 - 2  around structure  331 - 1  and  331 - 2 , are made through membrane  330 - 1  and  330 - 2  by dry etching. 
     FIG. 13  shows only on the first membrane  330 - 1  a layer of silicon dioxide  340  is deposited. After polishing to make the surface flat, the thickness of the dioxide layer  340  is the same as the depth of the recess  315 . Then the middle membrane  350  was deposited on the dioxide layer  340 . The structure of the middle membrane  350  is the same as that of the middle membrane  250  in  FIG. 9 . On the middle membrane  350  holes  351  can be mad around light path optionally. Holes  370 - 1  and  370 - 2  are etched through substrate  311 - 1  and  311 - 2  on the light path, respectively. 
     FIG. 14  illustrates the forming of first non-optical cavity  321 - 1  and first cavity  341 - 1  on the first substrate  311 - 1 , the second non-optical cavity  321 - 2  on the second substrate  311 - 2 , after etching by HF for certain time and fully cleaned and dried. 
     FIG. 15  shows the substrate  311 - 1  and  311 - 2  are bonded or glued face to face, forming the device  310 . At mean time, the second cavity  341 - 2  is formed due to the recess  315 . 
   For the device  210 , the first outer membrane  230 - 1  and the second outer membrane  230 - 2  can be doped on the first layer or the last layer of these membranes. The structure  231 - 1  and  231 - 2  may not be doped for the consideration of optical property of membranes. An adjustable voltage can be applied between the first substrate  211 - 1  and the first outer membrane  230 - 1 , the same adjustable voltage or another adjustable voltage can be applied between the second substrate  211 - 2  and the second outer membrane  230 - 2 . The membranes are attracted to the substrates and the first cavity and second cavity become thicker. When the two cavities vary at the same thicknesses, the devices can select different wavelength to pass through. The doping of layers and the voltage application are the same for the device  310  as those of device  210 . 
     FIG. 16  is a scheme of tunable filters adopting the device  210  or  310 . Light from a fiber  650  is coupled by a lens  651  to the device  310 . A lens  653  collects the passing through signal and couples it to a fiber  653 . 
     FIG. 17  is the scheme of add/drop multiplexers adopting the device  210  or  310 . For drop function, a two-fiber lens  662  coupled the light from a fiber  660  to the device  310  or  210 , light selected by the device  210 / 310  is collected by a lens  663  and is outputted by a fiber  664 . The rest light is reflected by the device and outputted by a fiber  661 . For add function, light can be inputted from the fiber  664  and the  661 , and is outputted from a fiber  660 . 
     FIG. 18  shows another scheme of add/drop multiplexers using a circulator  665 .