Abstract:
A tunable Fabry-Perot filter and a tunable vertical cavity surface emitting laser (VCSEL) are disclosed, where both devices utilize an improved dome structure for creating an internal air gap.

Description:
REFERENCE TO PENDING PRIOR PATENT APPLICATION  
       [0001]    This is a continuation-in-part of pending prior U.S. patent application Ser. No. 09/105,399, filed Jun. 26, 1998 by Parviz Tayebati et al. for MICROELECTROMECHANICALLY TUNABLE, CONFOCAL, VERTICAL CAVITY SURFACE EMITTING LASER AND FABRY-PEROT FILTER (Attorney&#39;s Docket No. CORE-33), which patent application is hereby incorporated herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to photonic devices in general, and more particularly to tunable filters and tunable lasers.  
         BACKGROUND OF THE INVENTION  
         [0003]    Tunable Fabry-Perot filters and tunable vertical cavity surface emitting lasers (VCSEL&#39;s) have recently generated considerable interest in the art. This is because these devices are believed to have application for a wide range of different optical components and systems, e.g., wavelength division multiplexing (WDM) fiberoptic systems, switches, routers, highly compact spectroscopic interferometers, optical trans-receivers, etc.  
           [0004]    In some tunable Fabry-Perot filters and in some tunable VCSEL&#39;s, tuning is achieved by using an electrostatic field to move a top mirror relative to a bottom mirror, whereby to change the length of the Fabry-Perot cavity and hence tune the wavelength of the device.  
           [0005]    While such a construction is advantageous in that it provides a fast and easy way to tune the device, in practice it has proven difficult to produce relatively uniform devices. Significant performance variations typically occur from device-to-device and from batch-to-batch.  
         SUMMARY OF THE INVENTION  
         [0006]    Accordingly, one object of the present invention is to provide an improved tunable Fabry-Perot filter.  
           [0007]    Another object of the present invention is to provide an improved method for fabricating a tunable Fabry-Perot filter.  
           [0008]    And another object of the present invention is to provide an improved tunable VCSEL.  
           [0009]    Still another object of the present invention is to provide an improved method for fabricating a tunable VCSEL.  
           [0010]    These and other objects are addressed by the present invention.  
           [0011]    In one form of the invention, there is provided a tunable Fabry-Perot filter which comprises a substrate, a bottom mirror mounted to the top of the substrate, a bottom electrode mounted to the top of the bottom mirror, a thin membrane support atop the bottom electrode, a top electrode fixed to the underside of the thin membrane support, a reinforcer fixed to the outside perimeter of the thin membrane support, and a confocal top mirror set atop the thin membrane support, with an air cavity being formed between the bottom mirror and the top mirror, and with the thin membrane support being in the form of a dome with openings therein, with the openings being small enough, and with sufficient distance therebetween, so as to substantially not affect the overall structural integrity of the dome, while still allowing chemical access to the region inside the dome.  
           [0012]    In another form of the invention, there is provided a tunable VCSEL which comprises a substrate, a bottom mirror mounted to the top of the substrate, a gain region mounted to the top of the bottom mirror, a bottom electrode mounted to the top of the gain region, a thin membrane support atop the bottom electrode, a top electrode fixed to the underside of the thin membrane support, a reinforcer fixed to the outside perimeter of the thin membrane support, and a confocal top mirror set atop the thin membrane support, with an air cavity being formed between the bottom mirror and the top mirror, and with the thin membrane support being in the form of a dome with openings therein, with the openings being small enough, and with sufficient distance therebetween, so as to substantially not affect the overall structural integrity of the dome, while still allowing chemical access to the region inside the dome.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    These and other objects and features of the present invention will be more fully disclosed or rendered obvious by the following detailed description of the preferred embodiments of the invention, which is to be considered together with the accompanying drawings wherein like numbers refer to like parts and further wherein:  
         [0014]    [0014]FIG. 1 is a schematic sectional view of a novel tunable Fabry-Perot filter formed in accordance with the present invention;  
         [0015]    [0015]FIG. 2 is a schematic sectional view of a novel tunable VCSEL formed in accordance with the present invention;  
         [0016]    FIGS.  3 - 11  are schematic views illustrating fabrication of the tunable Fabry-Perot filter of FIG. 1, wherein FIG. 3 shows a bottom mirror mounted to the top of a substrate and a bottom electrode mounted to the top of the bottom mirror, FIG. 4 shows a sacrificial structure mounted to the top of the bottom electrode, FIG. 5 shows the sacrificial structure after it has had its peripheral edges modified, FIG. 6 shows a top electrode deposited on the top of the sacrificial structure, FIG. 7 shows a thin membrane support deposited on top of the sacrificial structure, the top electrode and a portion of the bottom electrode, FIG. 8 shows a central aperture formed in the thin membrane support, FIG. 9 shows a reinforcer deposited about the periphery of the thin membrane support, FIG. 10 shows the top of the device after openings have been etched in the dome, and FIG. 11 shows a top mirror deposited on top of the thin membrane support. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Looking first at FIG. 1, there is shown a tunable Fabry-Perot filter  5  formed in accordance with the present invention. Filter  5  generally comprises a substrate  10 , a bottom mirror  15  mounted to the top of substrate  10 , a bottom electrode  20  mounted to the top of bottom mirror  15 , a thin membrane support  25  atop bottom electrode  20 , a top electrode  30  fixed to the underside of thin membrane support  25 , a reinforcer  35  fixed to the outside perimeter of thin membrane support  25 , and a confocal top mirror  40  set atop thin membrane support  25 , with an air cavity  45  being formed between bottom mirror  15  and top mirror  40 .  
         [0018]    As a result of this construction, a Fabry-Perot cavity is effectively created between top mirror  40  and bottom mirror  15 . Furthermore, by applying an appropriate voltage across top electrode  30  and bottom electrode  20 , the position of top mirror  40  can be changed relative to bottom mirror  15 , whereby to change the length of the Fabry-Perot cavity, and hence to tune Fabry-Perot filter  5 .  
         [0019]    Correspondingly, and looking next at FIG. 2, a tunable vertical cavity surface emitting laser (VCSEL)  50  can be constructed by appropriately modifying the construction of Fabry-Perot filter  5 , i.e., by positioning a gain region  55  between bottom mirror  15  and bottom electrode  20 . As a result of this construction, when gain region  55  is appropriately stimulated, e.g., by optical pumping, lasing can be established within air cavity  45 , between top mirror  40  and bottom mirror  15 . Furthermore, by applying an appropriate voltage across top electrode  30  and bottom electrode  20 , the position of top mirror  40  can be changed relative to bottom mirror  15 , whereby to change the length of the laser&#39;s resonant cavity, and hence to tune VCSEL  50 .  
         [0020]    If desired, thin membrane support  25  may be formed as a plurality of separate, relatively thin arms, and reinforcer  35  may be formed as corresponding peripheral posts.  
         [0021]    In general, forming thin membrane support  25  as a plurality of separate, relatively thin arms has at least two advantages: (1) it is easier to gain chemical access to the region below thin membrane support  25 , whereby to form air cavity  45 , and (2) it is easier to move top mirror  40  relative to bottom mirror  15  when an appropriate voltage is applied across top electrode  30  and bottom electrode  20 , whereby to tune Fabry-Perot filter  5  or VCSEL  50 .  
         [0022]    In practice, however, it has been discovered that forming thin membrane support  25  as a plurality of separate, relatively thin arms presents several problems. For convenience, these problems can be collectively referred to as problems of “noise”.  
         [0023]    More particularly, it has been found that separate, relatively thin support arms tend to vibrate with the mechanical shocks which are frequently encountered in the real world. Such vibrations can cause top mirror  40  to move relative to bottom mirror  15 , thereby causing Fabry-Perot filter  5  or VCSEL  50  to move in and out of “focus” or “tune”.  
         [0024]    Furthermore, as the power of Fabry-Perot filter  5  or VCSEL  50  rises, there can sometimes be a tendency for top mirror  40  to move upward relative to bottom mirror  15 , thereby causing the device to move out of “focus” or “tune”. In theory, the voltage applied to the device could be correspondingly increased so as to compensate for this effect and bring the device back into “focus” or “tune”, but in practice this has proven difficult to regulate. Furthermore, as the voltage applied to the device in increased, the curvature of top mirror  40  can change as well, thereby introducing new problems with device performance.  
         [0025]    It has now been discovered that the larger the surface area of thin membrane support  25 , and the stiffer it is, the better that the device can resist the “noise” problems described above. Accordingly, in accordance with the present invention, thin membrane support  25  is preferably fabricated in the form of a dome with openings therein, with the openings being small enough, and with sufficient distance therebetween, so as to substantially not affect the overall structural integrity of the dome, while still allowing chemical access to the region inside the dome.  
         [0026]    In accordance with the present invention, a Fabry-Perot filter  5  (FIG. 1) may be formed as follows.  
         [0027]    First, starting with a substrate  10  (FIG. 3), a bottom mirror  15  is mounted to the top of the substrate, and then a bottom electrode  20  is mounted to the top of bottom mirror  15 . Substrate  10  preferably comprises a semiconductor material such as Si, GaAs, InP or other suitable materials. Bottom mirror  15  preferably comprises a distributed Bragg reflector (DBR) formed out of alternating layers of quarter-wavelength thick deposited dielectric films, e.g., silicon (Si) and aluminum oxide (Al 2 O 3 ), or silicon (Si) and silicon dioxide (SiO 2 ), or silicon (Si) and magnesium oxide (MgO), or TiO 2  and SiO 2 , or Ta 2 O 5  or zirconium oxide, etc. Bottom mirror  15  may be deposited on substrate  10  by any suitable thin film deposition techniques. Bottom electrode  20  includes a central aperture  58 .  
         [0028]    Next, a sacrificial structure  60  (FIG. 4) of polyimide, or aluminum, or some other sacrificial material, is deposited on top of bottom electrode  20  (and, in the region of central aperture  58 , bottom mirror  15 ). The sacrificial structure  60  will act as a sacrificial layer to be removed later in the fabrication process, as described in detail below. It should be appreciated that it is important to accurately control the thickness and lateral dimensions of sacrificial structure  60 . This is because the thickness of sacrificial structure  60  will determine the ultimate length of the air cavity  45  in the tunable Fabry-Perot device and, hence, the unbiased resonant wavelength of the device. On the other hand, the lateral dimension of sacrificial structure  60  will determined the voltage response of the device and the resonance frequency. Sacrificial structure  60  preferably has a circular configuration when viewed from the top (although it may, alternatively, have a polygonal configuration if desired). Sacrificial structure  60  may be deposited on bottom electrode  20  (and, in the region of central aperture  58 , bottom mirror  15 ) by evaporation or standard coating methods.  
         [0029]    An etch-mask is then used to pattern sacrificial structure  60  so as to leave a circular (or, alternatively, polygonal) disk-shaped deposit defining an outwardly slanted edge  65  on its etched perimeter (FIG. 5). Slanted edge  65  preferably extends at an angle of approximately 45 degrees to the top surface of bottom electrode  20 .  
         [0030]    Next, top electrode  30  is deposited on sacrificial structure  60  (FIG. 6). Top electrode  30  may be deposited directly on the top surface of sacrificial structure  60 , or top electrode  30  may be deposited into a recess formed in the top surface of sacrificial structure  60 , e.g., in the manner shown in FIG. 6. Top electrode  30  preferably has a washer-like configuration, i.e., it preferably has a circular outer perimeter and a circular inner hole.  
         [0031]    Thereafter, thin membrane support  25  (FIG. 7) is deposited over sacrificial structure  60 , top electrode  30  and a portion of bottom electrode  20 . Due to the structure of the underlying elements, thin membrane support  25  essentially has a dome configuration. Thin membrane support  25  comprises a material different than the material used to form sacrificial structure  60 . By way of example but not limitation, thin membrane support  25  may comprise silicon nitride or a metal, e.g., titanium-tungsten (TiW). Thin membrane support  25  may be deposited on sacrificial structure  60 , top electrode  30  and bottom electrode  20  by standard deposition techniques.  
         [0032]    In the case where thin membrane support  25  is formed out of a material which is not transparent, the center portion is removed (FIG. 8) so as to form an aperture  67 .  
         [0033]    Next, a reinforcer  35  (FIG. 9) made of metal (such as Al or TiW) or a hard dielectric (such as silicon nitride) is selectively deposited on the periphery of thin membrane support  25  so as to form an annular peripheral rim which essentially covers and supports the peripheral portion of thin membrane support  25 . The lateral dimension of reinforcer  35  is selected such that a thick metal rim extends from bottom electrode  20 , up over the sloped edge  65  of sacrificial structure  60 , and up onto the top of the structure, as indicated in FIG. 9. The thick reinforcer  35  (formed out of metal or a hard dielectric) provides robust support for thin membrane support  25  (formed out of silicon nitride or TiW) after the underlying sacrificial structure  60  has been removed (see below). Reinforcer  35  may be deposited on thin membrane support  25  (and, at the periphery of reinforcer  35 , bottom electrode  20 ) by standard deposition techniques.  
         [0034]    In essence, thin membrane support  25  comprises a thin dome structure, and reinforcer  35  comprises a thick rim support for the periphery of thin membrane support  25 .  
         [0035]    Using an etch-mask, a plurality of small openings  70  (only several of which are highlighted)(FIG. 10) are then formed by etching through thin membrane support  25 , down to the underlying sacrificial structure  60 . These openings  70  provide gateways for etchants to selectively remove the underlying sacrificial structure  60 , as will hereinafter be discussed in further detail. Openings  70  preferably have a circular configuration, although they may also have a polygonal configuration if desired.  
         [0036]    Circular openings  70  are formed small enough, and with sufficient distance therebetween, so as to substantially not affect the overall structural integrity of the dome structure of thin membrane support  25 , while still allowing chemical access to the region inside the dome.  
         [0037]    If desired, openings  70  may also be formed in reinforcer  35 . To the extent that openings  70  are formed in reinforcer  35 , these openings are sized and spaced so as to substantially not affect the structural integrity of the rim structure of reinforcer  35 .  
         [0038]    Next, a circular top mirror  40  is then selectively deposited at the center of thin membrane support  25  (FIG. 11). In one preferred form of the invention, top mirror  40  comprises a distributed Bragg reflector (DBR) formed out of alternating layers of quarter-wavelength thick deposited dielectric films, e.g., silicon (Si) and aluminum oxide (Al 2 O 3 ), or silicon (Si) and silicon dioxide (SiO 2 ), or silicon (Si) and magnesium oxide (MgO), or TiO 2  and SiO 2 , or Ta 2 O 5  or zirconium oxide, etc. Top mirror  40  may be deposited on thin membrane support  25  by thin film coating technology.  
         [0039]    Top mirror  40  is preferably curved. More particularly, top mirror  40  is preferably curved so that the curved top mirror  40 , in combination with the planar bottom mirror  15 , together form a confocal stable resonator with a well-defined, near-Gaussian mode structure. In one preferred form of the invention, top mirror  40  has a radius of curvature, with the radius of curvature being optimized so that the mode size of the cavity matches the size of the core of an optical fiber.  
         [0040]    To the extent that top mirror  40  is to assume a curved configuration in the completed device (e.g., as shown in FIGS. 1 and 11), an appropriate magnitude and type of strain is introduced into top mirror  40  during deposition of the top mirror. This may be accomplished by controlled changes in deposition temperatures or deposition voltages.  
         [0041]    Finally, an etchant is used to selectively remove sacrificial layer  60  and form air gap  45  (FIG. 1). This etchant is introduced to the area under thin membrane support  25  via openings  70 , and may comprise an oxygen plasma (in the case where sacrificial structure  60  comprises polyimide) or a CF 4  plasma (in the case where sacrificial structure  60  comprises aluminum). This releases thin membrane support  25  along with top mirror  40 . To the extent that top mirror  40  is formed with an appropriate magnitude and type of strain to result in the formation of a curved configuration, the removal of sacrificial structure  60  permits the top mirror to assume its desired curved configuration. Since wet chemistry is, preferably, not involved in removing sacrificial structure  60 , there is no risk of the released thin membrane support  25  collapsing due to surface tension.  
         [0042]    This completes the fabrication of a tunable Fabry-Perot filter.  
         [0043]    A tunable VCSEL (FIG. 2) may be formed in corresponding fashion by depositing a gain region  55  between bottom mirror  15  and bottom electrode  20 . Gain region  55  may comprise an InGaAsP/InGaAs multiple quantum well (MQW) structure. Gain region  55  may be deposited on bottom mirror  15  by MBE (molecular beam epitaxy) or MOCVD (metal organic chemical vapor deposition) methods, and bottom mirror  20  may be deposited on gain region  55  by MBE or MOCVD or other thin film coating techniques. Lasing can be achieved by photo-pumping with a separate pump laser having a wavelength that is highly absorptive within the gain spectrum of the gain medium used in gain region  55 .  
         [0044]    The present invention can also be used to produce a current-injected tunable VCSEL as well. In this situation, intra-cavity electrical interconnections are made to the p-i-n junction in the gain structure.  
         [0045]    It will be understood that the foregoing detailed description of the preferred embodiments of the invention has been presented by way of illustration and not limitation. Various modifications, variations, changes, adaptations and the like will occur to those skilled in the art in view of the foregoing specification. Accordingly, the present invention should be understood as being limited only by the terms of the claims.