Abstract:
An optical switch having an insulator under a heater element is disclosed. The insulator reduces the heat loss thereby making the switch more efficient. The insulator is fabricated embedded in the underlying substrate on which the heater and the optical intersection are fabricated. A method of fabricating the optical switch having an insulator is disclosed. A trench is etched on the substrate and filled with oxide or other suitable insulating material. Then, the heater and the optical intersection are fabricated above the insulator.

Description:
BACKGROUND 
     The present invention relates to the art of thermal activation optical switches. More particularly, the present invention relates to efficient thermal activation optical switches and the method of fabricating the same. 
     In the field of electronic and data communications, bandwidth demand is surging because of the rapidly increasing number of users, higher communications access speeds, longer connection times, and the use of rich media, such as audio, video, and high-resolution graphics. Optical networks, having greater bandwidth than traditional electrical networks, are becoming increasingly popular. 
     For switching optical signals (lights), optical switches based on liquid and bubble technique have been used. In these switches, multiple optical paths are placed in as a matrix on planar-light wave circuits (PLC), or wave-guides, crossing at several intersections, each intersection being a switch. At an intersection, the light travels through fluid with the same optical properties as the wave-guide. As a result, the light travels unimpeded through the intersection. 
     When the light needs to be rerouted to a new path, a bubble is created in the intersection. The bubble displaces the fluid and alters the optical properties of the intersection, causing the light to be reflected to the new path. The bubble is created by heating the fluid at the intersection and eliminated by removing the heat. The bubble can be generated and removed quickly providing a fast and reliable switching function. For example, the bubble can be generated in several microseconds. Further, the bubble can be sustained by maintaining the heat at the intersection. In fact, by maintaining the heat at the intersection, the switching of the light can be maintained for an indefinite period of time. The heat is typically provided by applying electrical current, or power, through a heating element, usually a high-resistance resistor. The heat is maintained by continuous application of the power through the heating element. 
     During the generation and the maintenance of the bubble, the heat dissipates to the surrounding material (especially to the substrate on which the heater is fabricated) and is lost. The heat dissipation and loss lead to several problems. First, the switch is inefficient. That is, much of the applied power is lost to the substrate instead of being used to heat the fluid. Second, the power required to generate and to maintain the bubble is greater than the power required to do so without the heat loss. Because of the heat loss, the switch in general and the heater in particular, has a relatively high power requirement. The high power requirement not only increases power costs, but it also increases component costs throughout the entire system. This is because the requirement necessitates the use of components that are able to handle the relative higher power. 
     Third, the dissipated heat adversely affects surrounding circuits. Fourthly, the heater has a relatively high power density because of the high power requirement. This leads to premature heater problems such as fusing. Fifthly, the high power, thus the current, requirement may required special high voltage high current on-chip circuit which is difficult to do. Finally, because the heat loss, the switch-on time is longer than the switch-on time without the heat loss. 
     Accordingly, there is a need for an optical switch that overcomes the problems discussed above. 
     SUMMARY 
     The need is met by the present invention. According to one aspect of the present invention, an optical switch has an insulator on a substrate. A heater is fabricated above the insulator of the substrate, and an optical intersection is constructed above the heater. 
     According to another aspect of the present invention, a method of fabricating an optical switch is disclosed. First, an insulator is fabricated. Then, a heater is fabricated above the insulator. Finally, an optical intersection is constructed above the heater. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side sectional view of an optical switch in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     As shown in the drawings for purposes of illustration, the present invention is embodied in an optical switch having an insulator on a substrate. A heater is fabricated above the insulator of the substrate, and an optical intersection is constructed above the heater. Because the insulator reduces the loss of heat, the switch is more efficient, less power is required to generate and to maintain a bubble, surrounding circuits are less affected, and the switch-on time is decreased. Moreover, since less power is applied to the heater, the reliability of the heater is increased. 
     Referring to FIG. 1, an optical switch  10  (the “switch  10 ”) illustrates one embodiment of the present invention. The switch  10  is fabricated on a substrate  12 . A typical substrate  12  is a Silicon (Si) based substrate  12 . An insulator  14  is on the substrate. Here, the prepositional word “on” is used, without limitation, as a function word to indicate position in contact with, supported by, in close proximity to, embedded in, built-in the substrate  12 . In fact, for the illustrated embodiment, the insulator  14  is embedded in the substrate  12 . The insulator  14  and the substrate  12  may be separated by a thin layer of whetting oxide  13 . 
     A heater  16  is fabricated above the insulator  14 . The heater  16  may be made from a high resistance resistor using such material as TaAl (Tantalum Aluminum), TaN (Tantalum Nitride), Ni (Nickel), Cr (Chromium), Polysilicon, or other suitable material. The heater  16  is typically fabricated within a heater circuit layer  18 . The heater circuit layer  18  may include multiple sub-layers, the sub-layers forming circuits to deliver power to the heater  16 . 
     In one embodiment, the insulator  14  covers substantially the same area of as the heater  16 . The insulator  14  has a thickness ranging from 1.7 to 15 microns. In one embodiment, the insulator  14  is between ten to twelve microns thick. The insulator  14  may be an oxide such as Silicon oxide or other suitable material having low thermal conductivity properties such as Silicon Nitride, Silicon Carbide, Aluminum Nitride, or Aluminum Oxide. 
     An optical intersection  20  is constructed above the heater  16 . In FIG. 1, the optical intersection is generally indicated by a dashed rectangular box  20 . The optical intersection  20  includes a first wave-guide  22  and a second wave-guide  24 , both having ends terminating at the intersection  20 . A third wave-guide (not shown), a fourth wave-guide (not shown), or both may also terminate at the intersection  20 . 
     The structure of the heater circuit layer  18  including the heater  16  and the structure of the optical intersection  20  including the third and the fourth wave-guides are known in the art. For example, see U.S. Pat. No. 5,699,462 issued on Dec. 16, 1997 to Fouquet et al. and U.S. Pat. No. 5,852,689 issued on Dec. 22, 1998 to Donald. Both of these patents are incorporated herein by reference. In particular, FIG. 1A of the &#39;689 patent illustrates an optical intersection including four wave-guides. 
     Continuing to refer to FIG. 1, the optical intersection  20  includes wave-guide substrate  26  surrounding the wave-guides  22 ,  24  and defining a liquid chamber  28  for housing liquid. The wave-guide substrate  26  may also define a channel  30  for the liquid to access the chamber  28 . 
     In the off state, the chamber  28  of the switch  10  is filled with the liquid having same optical properties as the wave-guide. Accordingly, optical signal (light) entering the chamber  28  (for example, from the wave-guide  22 ) passes through the chamber  28  unimpeded (and reaches, for example, the second wave-guide  24 ). In the on state, power is sent to the heater  16  to generate a bubble within the chamber  28 , the bubble giving the chamber  28  a different refractive index than the wave-guide. As a result, the light is reflected toward a different wave-guide, thus implementing the switching function. 
     The optical switch  10  may be fabricated as follows: First, the insulator  14  is fabricated on the substrate  12 . This is accomplished by etching a trench on the substrate  12 , filling the trench with insulation material (for example, Silicon oxide or other suitable material having low thermal conductivity properties such as Silicon Nitride, Silicon Carbide, Aluminum Nitride, or Aluminum Oxide), and planarizing. For example, the trench etch can be performed by a Silicon (Si) etch processes such as wet chemical or dry plasma including reactive ion etch. The filling process can be performed using a chemical vapor deposition (CVD) processes such as LPCVD (low pressure CVD), APCVD (Atmospheric Pressure CVD) oxide deposition, PECVD (plasma enhanced CVD) TEOS (tetraethylorthosilicate) deposition, or SOG (spin on glass). The surface planarization can be performed by either CMP (chemical and mechanical polishing) or photo resist etching back. A whetting oxide layer  13  may be formed after the trench etching but before the filling process. 
     The insulator may cover an area substantially similar to the area covered by the heater  16 . After the insulator  14  is fabricated, then the heater  16  is fabricated above the insulator  14 . Techniques to fabricate the heater  18  using a heater circuit layer  18  are known in the art. Finally, the optical intersection  20  is constructed above the heater  16 . Again, techniques to construction the optical intersection  20  is known in the art. 
     From the foregoing, it will be appreciated that the present invention is novel and offers advantages over the current art. The present invention results in a more efficient optical switch minimizing or eliminating various problems associated with inefficient optical switches such as heat loss and high power requirement. Although a specific embodiment of the invention is described and illustrated above, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. For example, differing configurations, sizes, or materials may be used to practice the present invention. The invention is limited by the claims that follow.