Abstract:
An approach for obtaining a transducer mirror structure made from silicon. The structure may have a center portion and a perimeter portion that have an attachment between them which is made flexible after certain etching between the two portions. The attachment may be a web of links or legs. A force applied to the center portion at one end of the structure may cause the center portion to move relative to the perimeter portion. A piezo electric transducer or actuator may be attached to apply the force. An oxide layer, a thin layer of silicon and a mirror may be formed on the other end of the structure. The web of links or legs between the center and the perimeter portions may be established with an RIE etch of gaps through the structure to the oxide layer and an undercutting of the gaps with a KOH etch.

Description:
BACKGROUND 
     The invention pertains to cavities and particularly to cavity mirrors. More particularly, the invention pertains to transducer mirrors. 
     SUMMARY 
     The invention is a silicon-based transducer-mirror structure. The transducer structure may have two portions made by etching a piece of silicon. One portion may be for holding a mirror and the other portion may be for mounting to another structure such as, for example, a cavity. An actuator may be attached to the transducer-mirror structure to cause movement of one portion relative to the other portion of the structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a diagram of a transducer mirror structure having a center portion for holding a mirror and a peripheral portion for mounting to another structure such as a cavity; 
         FIG. 2  is a diagram of a silicon structure that may permit an infrared light beam to pass through the structure because of the infrared transmittance of the silicon; 
         FIG. 3  is a diagram of a transducer mirror structure having a lateral hole for holding a driver or actuator in place with a side pin-like item; 
         FIG. 4  is a cutaway diagram of the structure of  FIG. 3 ; 
         FIG. 5  is a diagram of the transducer mirror structure having a seal on the outer portion of the structure for hermetic containment relative to another structure such as a cavity; 
         FIG. 6  is a diagram of the transducer mirror structure having a mirror formed on a surface of an inner portion of the structure; 
         FIG. 7  is a diagram of the transducer mirror structure showing the actuator side, inner portion supports and a hole for securing an actuator; 
         FIG. 8  is a diagram of the components for attaching the actuator to the transducer mirror structure; 
         FIG. 9  is a diagram showing the position of the actuator for attachment to the transducer mirror structure; 
         FIG. 10  is a diagram of the transducer mirror structure assembled with the seal, mirror and actuator; 
         FIG. 11  is a diagram with a mirror end view of the transducer mirror structure and actuator which are attachable to each other without a side pin-like item to hold the actuator in place relative to the transducer mirror structure; 
         FIG. 12  is a diagram with actuator end view of the structure of  FIG. 11 ; 
         FIG. 13  is a flow diagram of a fabrication process of the transducer mirror structure; and 
         FIG. 14  is a diagram of the transducer mirror structure attached to another structure. 
     
    
    
     DESCRIPTION 
     It is highly desirable to tune optical cavity mirrors such as those used in cavity ring down spectroscopy so that the path length in the cavity is an integral number of wavelengths, such as in light path  41  in structure  38  shown in  FIG. 14 . This may be done for an optical gyro by machining a mirror place and a read web structure out of glass. This structure may be expensive to make from glass, but it can provide parallelogram motion for keeping the mirror area and a respective mirror situated on the area facing one direction and being in line with an axis during the motion. The axis may be a normal or perpendicular relative to the surface of the area. Such structure may be made from silicon and have these motion properties. The structure may be designed to have other motion properties. The invention includes this structure made from silicon or like material. This structure may be used to adjust a light path length in a cavity. 
     A cavity ring down spectroscopy or system (CRDS) may be described in U.S. patent application Ser. No. 12/137,007, filed Jun. 11, 2008; U.S. patent application Ser. No. 11/770,648, filed Jun. 28, 2007; U.S. patent application Ser. No. 11/633,872, filed Dec. 4, 2006; and U.S. patent application Ser. No. 10/953,174, filed Sep. 28, 2004, now U.S. Pat. No. 7,145,165. U.S. patent application Ser. No. 12/137,007, filed Jun. 11, 2008; U.S. patent application Ser. No. 11/770,648, filed Jun. 28, 2007; U.S. patent application Ser. No. 11/633,872, filed Dec. 4, 2006; and U.S. patent application Ser. No. 10/953,174, filed Sep. 28, 2004, now U.S. Pat. No. 7,145,165, are hereby incorporated by reference. 
     The present device with similar qualities of a glass transducer mirror structure may be made out of silicon with appropriate processes. It may be a laterally translatable mirror structure. A combination of reactive ion etching and KOH etching may be used to define the legs or supporting links. A sacrificial layer such as that used on SOI wafer may be used to stop the etch on an SiO2 layer. This layer may later be removed or left as is. Thus, the front membrane may be the thickness of the SOI layer. The back web of legs or links may be formed out of a structure that is defined by a top silicon nitride pattern on the wafer which is later removed by chemical or plasma etching. 
     In other words, one might start with an SOI (silicon on insulator) wafer which has a Si layer on a thin oxide layer which in turn is on a thick Si layer. The front surface may be formed by etching spider legs “almost” all the way through the thick layer. The back legs which are on the side may be formed by etching up to the SiO 2  SOI etch stop. 
     The present device may be situated in a cavity block and optically sealed to the block. A piezo driver or other actuator may be mounted such that it pushes against the center part while the outside of the device forms a seal with the optically smooth cavity block surface at the mirror mount position. 
     The present silicon device may also be integrated with a block that is formed of silicon rather than glass. The potential for making smaller blocks at lower cost with low cost mirrors makes the CRDS more attractive to make for a number of applications. 
       FIG. 1  is a diagram or insulator (SOI) cavity ring down system or sensor (CRDS) mirror structure  10 . A layer  11  may be silicon on one side of an SiO 2  insulator  12 . Also a silicon layer  13  may be on the other side of SiO 2  insulator layer  12 . A mirror layer may be formed on layer  13 . Layer  13  may have a thickness of, for instance, about 30 microns. The layers  11 ,  12  and  13  may be designed with various thicknesses as appropriate for a respective application of structure  10 . 
     Layer  11  may be etched down in selected spots to the SiO 2  layer  12  with a deep RIE as shown with arrow  14 . Also, a KOH etching approach may be used to etch under the legs  15  in that the etching causes an undercut as shown by arrow  16 . RIE may etch or cut straight ahead whereas KOH tends to undercut. Legs  15  may of various sorts of shapes, so long as pressure on one side of the structure  10  moves a mirror on the other side of the structure along a straight or other designed optical path relative to structure  10 . 
     The structure  10  may have a low RMS surface roughness. There may be a membrane or seal around the perimeter of the transducer-mirror structure  10  so that it may be hermetic when the structure  10  is inserted on or in an opening or installed as part of another structure such as, for instance, a cavity. Structure  10  may include layers  12  and  13  which hold and support the mirror. 
     A central portion of surface  13  on which the mirror is formed may be moved relative to the outside portion of structure  11  with a pressure  17  exerted against area  18 . Such pressure  17  may be effected with a piezo electric actuator or other mechanism. The mirror on the external surface of layer  13  may be a ring laser gyro (RLG) like mirror.  FIG. 6  shows an example location of a mirror  23  on structure  10 . Mirror  23  may, for example, be made with a deposition of alternating dielectric materials of a quarter-wave thickness. The alternating layers of materials may include Si and SiO 2  or some other high and low index materials. These materials may be deposited at the center area of a substrate or structure  10  on the side opposite of the side for an actuator. 
     Structure  10  may be modeled after RLG mirrors with SiO 2  drilling. Short-wavelength infrared (SWIR) laser transmission through the silicon may be had, if desired, as shown by an infrared light  19  in  FIG. 2 . Often, the structure is not necessarily designed to permit light  19  to go through the transducer mirror structure  10 . In some designs, there may be sufficient leakage of light  19  for monitoring purposes. 
     Use of silicon for structure  10  may lead to low cost fabrication. Another approach for making a structure  10  that may achieve the purpose of structure  10  may be a use of glass in lieu of silicon. To make and machine glass to achieve such a structure could be much more expensive to fabricate than the silicon structure  10 . Significant savings may be achieved by making numerous structures  10  from silicon at a wafer level. 
       FIG. 3  is a diagram which shows a version of structure  10  with a hole  21  for holding a piezo electric driver with a pin on the side of the structure.  FIG. 4  is a cutaway diagram of the structure shown in  FIG. 3 .  FIGS. 8-10 , as described herein, show further information of this approach for securing an actuator or driver to a transducer mirror structure. Another approach without a side pin is shown in  FIGS. 11 and 12 . 
       FIG. 5  is a diagram of structure  10  shown with a seal  22  on the surface near an outer or peripheral portion so as to hermetically contain a mirror within another structure such as a cavity. The seal could, in some instances, be on the outside edge of structure  10 .  FIG. 6  is a diagram of structure  10  in  FIG. 5  but additionally having a dielectric mirror  23  formed or deposited on the surface of the inner or center portion of structure  10 .  FIG. 7  is a diagram of structure  10  with gaps and resulting supports  29 , e.g., legs, links, adapted or etched, and a hole  28  for attaching an actuator such as piezo electric driver on structure  10 .  FIG. 8  is a diagram of components for implementing an actuation mechanism for structure  10 . Component  24  is an actuator such as, for example, a piezo electric driver. Component  25  is a pin or like item that may be placed in hole  21  of structure  10  and hole  26  of a shaft or pin  27  attached to the actuator or driver  24 . 
       FIG. 9  is a diagram of the side of structure  10  upon where the actuator or driver  24  may be attached with short pin or shaft  27  inserted into a hole  28  of structure  10 . Etched gaps with resulting thin supports  29  for center portion of structure  10  are shown on the actuator side.  FIG. 10  is a diagram of the mirror side of structure  10  with the actuator or driver  24  attached to structure  10  and held in with pin  25  inserted through hole  21  of structure  10  and hole  26  of pin or shaft  27  of actuator or driver  24 . Glue may be applied to pin  25  and/or hole  21  for sealing to prevent gas from escaping from the cavity through any portions of hole  21 . An indication  37  may show a direction of mirror  23  motion relative to the perimeter of structure  10  such as at the seal  22 . Line  39  may provide control of actuator  24  which is for providing the motion according to indication  37  to mirror  23 . The motion of mirror  23  may be for adjustment of mirror position. 
       FIG. 11  is a diagram with a mirror end view of the transducer mirror structure  10  and driver or actuator  24 , such as a piezo electric driver, which are attachable to each other without a side pin-like item  25  like that in  FIG. 10 , to hold the actuator  24  in place relative to the transducer mirror structure  10 .  FIG. 12  is a diagram with actuator  24  end view of the structure of  FIG. 11 . The main attachment may be the pin or shaft  27  in  FIG. 11  on the actuator  24  being glued to the back of the silicon transducer mirror structure  10 , where pin or shaft  27  fit into hole  28  in structure  10  as shown in  FIG. 12 . Actuator  24  may be sealed to the backside of structure  10  so that any gas would not escape from the cavity through the structure  10  openings. The front side of the cavity may be sealed to the structure around the periphery. The external perimeter of structure  10  might be circular rather than a square cut. Hole  28  may be etched in the silicon structure  10  at the same time that the other openings are made in the structure. 
       FIG. 13  is a flow diagram of a fabrication process of the transducer structure  10 . Step  31  may be obtaining a silicon piece, block or substrate for structure  10  which may be etched to form gaps, slots, links, legs or supports  29  to hold a center portion  35  relative to an outer portion  36 . Step  32  may include a straight RIE etch for making, for instance, gaps for the flexible supports  29 . Step  33  may include a second etch which is a KOH under etch for further making the supports  29 . Step  34  may include making a hole  28  for holding an actuator. Gaps, slots, links, legs and supports  29 , and the like, may be of various shapes. Other steps may include adding a mirror  23  and a seal  22  as described herein, and attaching resultant structure  10  to another structure  38  shown in  FIG. 14 , such as a cavity of a cavity ring-down spectroscopy system. 
     In the present specification, some of the matter may be of a hypothetical or prophetic nature although stated in another manner or tense. 
     Although the invention has been described with respect to at least one illustrative example, many variations and modifications will become apparent to those skilled in the art upon reading the present specification. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.