Patent Publication Number: US-2003228089-A1

Title: Method of controlling the curvature of an optical device

Description:
FIELD OF THE INVENTION  
       [0001] The invention relates generally to optical devices and, more particularly, the invention relates to controlling the curvature of an optical networking device that either or both reflects and transmits light.  
       BACKGROUND OF THE INVENTION  
       [0002] Optical networks are becoming the data transmission medium of choice in the networking field. Among other advantages, optical networks generally have a higher bandwidth and lower power/line loss. To that end, optical fibers carrying data typically connect with an optical switching device that reflects incoming light signals between fibers. More specifically, optical switching devices typically have one or more internal mirrors that reflect light beams between optical fibers.  
       [0003] To accurately reflect light beams between different optical fibers, each mirror must have a well controlled radius of curvature. Data can be lost by misdirected and/or dispersed light beams if the curvature is off by a relatively small amount. Accordingly, one currently used technique to control the radius of curvature of a mirror applies a film, with a known coefficient of thermal expansion, to a silicon mirror base. Since the coefficients of thermal expansion are known for both the film and the silicon mirror base, the radius of curvature can be controlled to some extent by selecting the appropriate material as a film, and manipulating other design parameters.  
       [0004] Although sufficient in some instances, it would be advantageous to more accurately control the radius of curvature of optical mirrors. It also would be advantageous to permit use of films that do not have the coefficients of thermal expansion required to achieve the correct radius of curvature.  
       SUMMARY OF THE INVENTION  
       [0005] In accordance with one aspect of the invention, a method of controlling the curvature of an optical device applies a material that affects the molecular structure of the optical device. To that end, a material having at least one predetermined property is selected, and then applied into the optical device. Application of the material causes stress in the optical device. The curvature of the optical device thus changes after the material is applied. As noted above, the material affects the molecular structure of the optical device.  
       [0006] The optical device may be at least one of a mirror and a lens. In some embodiments, the optical device includes a surface to receive the application, where the material is applied to less than the entire area of that surface. The material is comprised of molecules having a size. The stress in the optical device thus is a function of the size of the molecules of the material. In addition, the optical device may include silicon.  
       [0007] The optical device may include a surface to which a reflective layer is applied. In addition, the optical device may be heated to further affect the molecular structure of the optical device. By way of example and not limitation, the material may be one of phosphorous, boron, arsenic, germanium, carbon, indium, and aluminum. In some embodiments, the material is applied via at least one of a diffusion process and an implantation process.  
       [0008] In accordance with another aspect of the invention, a method of setting the curvature of a light distorting portion of an optical device to a given curvature selects a material having at least one predetermined property, and provides the light distorting portion. The method also applies the material to the light distorting portion based upon at least one predetermined property of the material. The curvature of the light distorting portion changes to the given curvature after the material is applied. In illustrative embodiments, the material affects the molecular structure of the light distorting portion.  
       [0009] Among other things, the light distorting portion may be at least one of a silicon lens of a MEMS device and a silicon mirror of a MEMS device. In illustrative embodiments, the material is a charged material that interacts with the light distorting portion to cause the curvature of the light distorting portion to change from its unapplied state. The method further may heat the light distorting portion after the material is applied.  
       [0010] In accordance with other aspects of the invention, a method of manufacturing a reflective mirror of an optical device shapes the reflective mirror to a given curvature. To that end, the method provides a substantially flat silicon mirror base, and selects a material having at least one predetermined property. In addition, the method applies the material to the mirror base as a function of at least one property of the material. The curvature of the mirror base changes to the given curvature after the material is applied. The material affects the molecular structure of the light distorting portion. The method continues by adding a reflective layer to one surface of the mirror base.  
       [0011] The mirror may be heated after the material is applied. In some embodiments, the reflective layer is added after the mirror base is heated. In other embodiments, the reflective layer is added before the mirror base is heated. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0012] The foregoing and advantages of the invention will be appreciated more fully from the following further description thereof with reference to the accompanying drawings wherein:  
     [0013]FIG. 1 schematically shows an exemplary network that may be used with illustrative embodiments of the invention.  
     [0014]FIG. 2 schematically shows an exemplary optical switch that may use mirrors produced in accordance with illustrative embodiments of the invention.  
     [0015]FIG. 3 shows a process of producing a mirror in accordance with illustrative embodiments of the invention.  
     [0016]FIG. 4A schematically shows an exemplary material as it is initially applied to a silicon base in a first manner.  
     [0017]FIG. 4B schematically shows the silicon base of FIG. 4A after the material is applied.  
     [0018]FIG. 5A schematically shows an exemplary material as it is initially applied to a silicon base in a second manner.  
     [0019]FIG. 5B schematically shows the silicon base of FIG. 5A after the material is applied.  
     [0020]FIG. 6 shows a process of producing a lens in accordance with illustrative embodiments of the invention. 
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS  
     [0021] In illustrative embodiments of the invention, a silicon mirror or lens is produced by applying a material directly into the crystal lattice so that it bows in a controlled manner. By using this technique, a mirror or lens may be precisely shaped to more effectively control a beam of light within an optical device, such as an optical micro-electromechanical system (“MEMS”). Details are discussed below.  
     [0022]FIG. 1 schematically shows an exemplary network  10  that may use optical switches  12  having mirrors or lenses (FIG. 2) produced in accordance with illustrative embodiments of the invention. The network  10  includes three switches  12  that connect between two local area networks  14  and the Internet  16 . At least one of the switches  12  includes mirrors or lenses for redirecting light beams received over a fiber optic cable. It should be noted, however, that discussion of the network configuration shown in FIG. 1 is exemplary and not intended to limit the scope of the invention. Accordingly, other network configurations at least in part using light transmission media may be used.  
     [0023]FIG. 2 schematically shows an exemplary switch  12  that may be used in the network  10  of FIG. 1. The switch  12  includes an input port  18  for receiving a light beam via a fiber optic cable  22 , and an output port  20  for transmitting the light beam via another fiber optic cable  22 . Although multiple fiber optic cables  22  may be coupled with the switch  12  via multiple ports, only a single input and output port  18  and  20  are shown for simplicity. The switch  12  also includes three mirrors  23  to reflect the incoming light beam from the input to the output. In illustrative embodiments, the switch  12  is a micro-electromechanical system (“MEMS”). Of course, various embodiments are not limited to switches. Other network devices or light processing devices that use a mirror and/or a lens may incorporate illustrative embodiments of the invention.  
     [0024] As suggested above, illustrative embodiments may be applied to either a lens or a mirror  23 . FIG. 3 shows a process of producing a mirror  23  in accordance with various embodiments of the invention the process begins at step  300 , in which a silicon base (shown in FIGS. 4A, 4B,  5 A, and  5 B, and identified by reference number  24 ) is produced. To that end, a single silicon wafer is divided into a plurality of individual silicon bases  24 . Each base  24  is to ultimately become a mirror  23 . In some embodiments, each mirror  23  also is manufactured to include springs and/or gimbals. At this point in the process, each base  24  is substantially flat.  
     [0025] The process continues to step  302 , in which a material is applied to the silicon base  24  to cause it to bow either in a concave or convex manner. As known by those skilled in the art, the mirror  23  should be concave on its reflecting side. Accordingly, if the material is applied to the reflective side of the base  24 , then it should have properties to cause that side of the base  24  to bow in a concave manner. Alternatively, if the material is applied to the nonreflective side of the base  24 , then it should cause that side of the base  24  to bow in a convex manner.  
     [0026] Before it can be applied, however, properties of the material must be determined. Specifically, before applying the material, the mirror designer must select the final radius of curvature of the finished product. Specific materials thus are analyzed to determine which material is most appropriate to attain that final radius of curvature. To that end, the material properties are analyzed to determine the amount of stress the material will exert on the base  24 . These stresses should cause the base molecules to either contract or expand. One property of importance thus is the physical space that the atoms of the material would take from the molecules in the silicon lattice. Accordingly, in addition to the properties of the material, the molecular structure of the base  24  also should be taken into account.  
     [0027] Among others, the material can include charged particles, such as ions. Exemplary ions that may be used include phosphorous, boron, and arsenic. Alternatively, elements known to cause stress, such as germanium, carbon, indium, and aluminum also may be used. Note that these materials are discussed herein as exemplary materials and not intended to limit the scope of the invention. Other materials thus may be used. In a similar manner, some embodiments are not limited to a base  24  produced from silicon. Accordingly, bases produced from other materials may be used in some embodiments.  
     [0028] If the material is applied from the non-reflective side of the base  24 , then a material having properties that cause the reflective side to bow into a concave shape should be selected. In such case, phosphorus atoms may be used. Of course, if the material is applied from the reflective side of the base, then another material should be used. Note that this discussion is not limited to shallow ion applications. In some applications, for example, the material passes through most of the thickness of the base  24 . Such a material may be a light specie with high energy, such as boron.  
     [0029] Various types of material initially damage the lattice of the silicon when applied. In particular, a given ion implanted within the base  24  can create mechanical stress by damaging the crystal lattice of the silicon. Different materials, however, damage the crystal lattice to different extents. Accordingly, two or more different types of material may be used on the same base  24 . In particular, a first material may be used as a course tuning material to get the base  24  to a specified curvature range. After in that range, one or more other materials may be used to fine tune the curvature to very specific tolerances. For example, phosphorous causes more lattice damage and thus, causes more bowing than boron. Boron thus may be used to fine tune an initial phosphorous application.  
     [0030] The material may be applied to the base  24  a number of different ways. For example, the material may be applied by conventional ion implantation techniques. Alternatively, the material may be applied by conventional diffusion techniques. Yet other types of techniques may be used, such as homoepitaxy and heteroepitaxy films with suitable stress.  
     [0031] Additional benefits may be derived by applying the material to the base  24 . For example, the conductivity of the silicon may increase. Because mirrors  23  commonly are rotated by electrostatic forces within the switch  12 , increased conductivity can improve switch performance.  
     [0032] If the process ended at step  302 , then the radius of curvature would be controlled solely by damage to the crystal lattice. In fact, if the process ended at step  302 , then instead of being a mirror  23 , the base  24  could be a lens because silicon transmits infrared light. Specifically, fiber optic cables  22  transmit data in infrared data signals, which generally cannot be reflected by silicon to a useful extent. Moreover, the lens would have its radius of curvature controlled merely by damage to its crystal lattice.  
     [0033] The base  24  may be further processed, however, by causing the damaged crystal lattice to regenerate, thereby causing the base  24  to continue to controllably bow (in either direction). The regeneration process may be referred to as “substitution.” Accordingly, the process continues to step  304 , in which it is determined if it is desirable to regenerate the crystal lattice. If it is desirable, then the base  24 , which at this point already has been processed by the material application, is heated to a specified temperature (step  306 ). In many cases, a high temperature anneal on the order of 800-900 degrees centigrade should promote satisfactory lattice regrowth. Again, in a manner similar to step  302 , if the process ended, then the base  24  would be a lens and not a mirror  23 .  
     [0034] The process then continues to step  308 , in which a reflective film is added to one surface of the base  24  in accordance with conventional processes, thus ending the process. In the optical network application discussed herein, the reflective surface may be any material that effectively reflects infrared light. Among other materials, gold produces satisfactory results.  
     [0035] It should be noted that various steps in the discussed process can be executed at different times. For example, some material used for the reflective surface may be able to withstand high temperature anneals. Accordingly, when such materials are used, the reflective film may be added to the base  24  before the heating step. Conversely, when using a gold film, the heating step must be executed before the film is added because the maximum anneal temperature of gold is much lower than 800-900 degrees centigrade.  
     [0036] In a manner similar to the silicon base  24 , the reflective film also has an associated coefficient of thermal expansion. Accordingly, the coefficient of thermal expansion of both the film and the base  24  should be taken into consideration when selecting the appropriate materials to apply into the silicon. In illustrative embodiments, however, the respective coefficients of thermal expansion of the base  24  and the film have a negligible effect on the final curvature of the mirror  23 . Instead, the properties of the applied material provide the overriding bowing effect.  
     [0037]FIGS. 4A and 4B schematically show the bowing effect caused by the applied material. FIG. 4A shows the silicon base  24  just prior to material application. In such state, the silicon base  24  is substantially flat. The material is shown in FIG. 4A as being substantially evenly applied to the silicon base  24 . Damage to the crystal lattice of the base  24  therefore is substantially uniform. Accordingly, as shown in FIG. 4B, the silicon base  24  bows substantially uniformly.  
     [0038]FIGS. 4A and 4B show the case when the silicon base  24  is damaged only. No regrowth occurs in such case. As discussed above, if heated, the silicon base  24  should begin to regrow. The regrowth process can cause the silicon base  24  to further bow in either direction, depending upon the material and silicon base properties.  
     [0039] In some embodiments, only selected portions of the silicon base  24  are controllably bowed. FIGS. 5A and 5B schematically show an example of such embodiments. Specifically, the material may be applied in a higher concentration in selected portions of the silicon base  24 , thus causing the base  24  to bow more where more material was applied. In fact, many different portions of the silicon base  24  can have a different radius of curvature, consequently bowing in different directions. To that end, either the same or different materials may be applied to specified parts of the base  24  in varying concentrations.  
     [0040]FIG. 6 shows a process of producing a lens in accordance with illustrative embodiments of the invention. This process is very similar to the process of producing a mirror  23  discussed with regard to FIG. 3, except it does not require a reflective film (step  308 ). In some embodiments, however, an antireflective film is added to both sides of the lens. Accordingly, in summary, the process begins at step  600 , in which a silicon base  24  is produced. A material then is applied to the base  24  at step  602 . It then is determined at step  604  if regenerative processes are to be used. If not, the process ends. Conversely, if regenerative processes are to be used, then the process continues to step  606 , in which the processed base  24  is heated, thus ending the process. Note that many of the details discussed above with regard to the mirror  23  apply to the lens. This brief discussion of the lens thus is intended to be cursory in view of the details noted above with regard to the mirror technique.  
     [0041] Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made that will achieve some of the advantages of the invention without departing from the true scope of the invention. These and other obvious modifications are intended to be covered by the appended claims.