Abstract:
Fast silicon diodes and arrays with high quantum efficiency built on dielectrically isolated wafers. A waveguide is formed in the top surface of the silicon that utilizes total internal reflection from the Si—Si Oxide interface to form an internal mirror. This mirror reflects incoming light into the waveguide cavity, with the light being trapped there by surrounding reflective interfaces. A masking layer may be used to define an input window. Individual diodes or linear arrays may be formed as desired. Some alternate embodiments are described.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Patent Application No. 60/560,881 filed Apr. 9, 2004. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to semiconductor photodiodes, in particular to high frequency, broad spectral range, silicon photodiodes. 
   2. Prior Art 
   A growing demand in high-speed photodetectors stimulated creation of a new generation of photodiodes capable of working in NIR (near IR) spectral range with close to 100% quantum efficiency (QE).  FIG. 1  represents traditional structures, using single-pass normal incidence absorption regions that are coupled to the transit time, and are not able to decouple the requirements of speed and efficiency from the design parameters of depletion volume and working voltage. Novel approaches may be used to achieve wider bandwidth of over 30 Gb/s with 100% QE using III-V hetero-structures but are not applicable to main stream production. Intensive studies toward creating very fast and highly sensitive photodiodes were performed during the last several years with III-V compound structures based on InP—GaInAsP. The ideas explored include but are not limited to side-illuminated photodiodes, evanescently coupled photodiodes, and those with either an integrated taper or graded-index waveguides. 
   The idea of creating a waveguide-type Si photodiode is very attractive because it offers a relatively inexpensive, novel design that would be useful in many applications. A few designs based on a waveguide-type Si structures are known in the literature.  FIGS. 2A ,  2 B and  2 C present a lateral waveguide detector using a diffraction principle to direct the radiation into a top surface waveguide, while  FIG. 3  presents a light trap to capture the radiation in the detector&#39;s sensitive region. These designs make use of highly reflective distributed Bragg reflectors (DBR) fabricated using a commercially available double-SOI process, internal total reflection of V-grooves made on the chip back surface, buried reflecting mirror, lateral pin photodiode structure with alternating p-type and n-type doped fingers, and a waveguide-grading-coupler built on the surface of a planar Si photodiode with a buried oxide layer. Each of those photodiodes has drawbacks that limit their application. 
   The following more detailed discussion of the prior art assumes a p-on-n structure; it is understood that the same device can be made with an n-on-p structure where the names of the anodes and cathodes reversed. Referring now to the drawings,  FIG. 1  shows a prior art photodiode with a depletion region  40  designed to separate and capture the generated electron hole pairs. This region is depleted by bias applied to the anode  10  and cathode  20  of the device. The top passivation glass  30  and a metal contact  50  are also shown. As the wavelength increases and the associated absorption length increases, the depletion region  40  must be made deeper requiring more voltage or higher resistivity material to support the depletion. This then presents a diode, which is optimized for the selected wavelength. It is desirable to provide a technique to eliminate this wavelength dependence. The elimination of this dependence points towards a surface region waveguide type of detector. 
     FIGS. 2A ,  2 B and  2 C show an attempt to accomplish a waveguide design for the p-on-n structure.  FIG. 2A , top view, shows the anode  10  and cathode  20  with the active waveguide region  40  located between them.  FIGS. 2B and 2C  shows side cross-sections, through the anode and cathode respectively, in which the isolation oxide (or buried Bragg mirror)  30  and substrate support  60  are apparent. The issue is to get the light to enter into the top surface of the silicon, and remain there until it is converted into an electrical signal. This detector utilizes a diffraction technique to bend the light wave as it enters the surface of the silicon, so that it moves laterally along the material surface in the sensitive region of the detector. The issue here is that the diffraction grid  55  is both inefficient and wavelength dependent. 
     FIG. 3  presents another attempt to form a surface detector. It focuses upon the use of repeated reflections at an angle that invokes total internal reflection; hence the light becomes trapped in the surface of the silicon. The issue here is the topside electrode and mirror  70  causes a loss due to surface reflection before the light enters the silicon. This patent teaches that a KOH etch will form grooves in a &lt;100&gt; wafer by selectively etching along a preferential crystalline plane ( 100 ) forming an angle of ˜54.7°, which is the angle between the [111] and [100] directions. The material  35  is an optical adhesive bonding the silicon  40  to a substrate wafer  60 . 
   The present invention also has its own limitations; however, it out performs the designs mentioned above with respect to a number of important parameters. Most importantly, this design is relatively easy to fabricate in Silicon using dielectrically isolated (DI) wafers similar to those used for photovoltaic cell fabrication. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The main ideas of the invention as distinguished from the prior art are demonstrated by the accompanying drawings, wherein: 
       FIG. 1  is a simplified schematic cross section of a typical, conventional structure photodiode using single-pass normal incidence absorption regions that are coupled to the transit time. 
       FIG. 2A  is a simplified schematic of a waveguide photodetector after Csutak et al. showing a top view without the diffraction coupler. 
       FIG. 2B  presents the waveguide photodetector of  FIG. 2A  in side view cross-section through the anode, with the diffraction coupler. 
       FIG. 2C  presents the waveguide photodetector of  FIG. 2A  in side view cross-section through the cathode, with the diffraction coupler. 
       FIG. 3  is a simplified schematic of an infrared detector using a light trapping structure to increase efficiency after Chen et al. 
       FIG. 4  is a top view of the present invention. 
       FIG. 5  is a side view in cross section of the present invention taken along line  5 - 5  of  FIG. 4 . 
       FIG. 6  shows a ray trace of the light within the waveguide. 
       FIG. 7  presents a single element of an array with the bond pad attached. 
       FIG. 8  is an array made of elements shown in  FIG. 7 . 
       FIG. 9  presents typical performance and design values for a 20 micron structure design. 
       FIG. 10  presents typical performance and design values for a 30 micron structure design. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In the present invention, dielectrically isolated (DI) wafer process techniques are used to form a waveguide in the top surface of the silicon that utilizes total internal reflection from the Si—Si Oxide interface to form an internal mirror. This mirror reflects incoming light into the waveguide cavity and traps it there.  FIG. 4  is a top view of the preferred embodiment of the invention showing a clear unobstructed entrance aperture  140  at one end of the device. The waveguide runs parallel to this top surface and includes the anode diffusion  160  of the p-on-n structure; this would be the cathode for the case of n-on-p structure. The silicon oxide  120  delineates the extent of the waveguide, sides, bottom, ends and top, and is surrounded by a supporting material  110  such as poly-silicon. This material, if conductive, can be used to form an electrical path from the contact  130  to the backside of the wafer, or if a dielectric material is used the contact can be formed on the front side by adding bond pads. It should be noted that it is possible to attach a substrate wafer for strength and handling purposes in which case the layer  110  would be an adhesive over the substrate wafer. Depositing poly silicon and polishing it could also facilitate the use of a direct bonded substrate for handling purposes. Whatever is used, whether a deposited layer or an adhesive layer bonded to a substrate shall be referred to in the claims as a body to distinguish from the silicon regions forming the photodiodes. 
   The lateral dimension “b” is not coupled to any of the waveguide performance characteristics, and can be selected as needed for the incoming light. The dimension “a” is coupled to the depth “h” of the waveguide as can be seen in  FIG. 5 . Increasing “a” requires a deeper waveguide to assure that the beveled mirror surface is beneath the aperture.  FIG. 5  also shows the optional masking layer  190  that is normally added to eliminate any stray light thereby reducing this source of noise. 
   The waveguide includes the active silicon volume  170 , which is normally fully depleted when biased in use, the anode (cathode)  160  and the cathode (anode)  180 . In the preferred embodiment, it is fully encased in silicon oxide except for the contacts  130  and  150 .  FIG. 6  shows a ray trace within the waveguide, demonstrating trapping of the light within the active volume. Note that the waveguide construction is, to first order, independent of the wavelength thereby providing a broad spectral range of usefulness. Second order effects associated with the wavelength vary the onset angle for total internal reflection; this variation is small and is normally not a design issue. 
   Thus in the preferred embodiment, the effect of a total internal reflection of light from Si—Si oxide interface is exploited. Note that for this interface, the total internal reflection angle is equal to approximately 24 degrees for the NIR spectral range. Therefore, the total internal reflection condition is fulfilled for the V-groove type structure built in this work. 
   Light penetrates the opening aperture of the photodiode and reflects totally from the V-groove wall due to the total internal reflection effect (see  FIG. 6 ). Several multiple total internal reflections follow which facilitate loss-free light propagation inside the waveguide, allowing absorption of the entire optical radiation signal. The lateral light propagation is de-coupled from the transverse motion of non-equilibrium carriers, thereby eliminating the requirement of a deeper depletion for a longer wavelength. Under conditions of total depletion of the waveguide depth “h”, the transit time of non-equilibrium carriers is governed by the electric field only. By properly selecting the starting material resistivity and the reverse bias value, the carriers&#39; transit velocity can be saturated and their drift time minimized. 
   The structure should be designed in a way such that the “rear” surface angle of incidence α is larger than the total internal reflection angle of approximately 24 degrees—(see  FIG. 6 ). Such a design will optimize the sensitivity of this photodiode in near infrared spectral range. It is obvious that to satisfy the above condition, the overall length of the waveguide may be within certain allowed ranges, specific for each depth of the waveguide and opening aperture size. 
   The main characteristics of an exemplary design are summarized in  FIGS. 9 and 10  for wave-guide depths of “h”=20 um and 30 um, respectively. Note that the length of the waveguide is not specified in these Figures. 
     FIGS. 9 and 10  compare characteristics of some potentially useful p-on-n and n-on-p structures. Note that due to a trade-off between the electric field value at a certain reverse bias and Si resistivity from the one side, and the carrier velocity from the other side, a very short value of the time response and, correspondingly, a high frequency bandwidth could be obtained for both the p-type and n-type material. The cut-off frequency will be higher than the frequency bandwidth shown in  FIGS. 9 and 10 . 
   Note also that the values of responsivity and quantum efficiency (QE) given in  FIGS. 9 and 10  were calculated assuming no loss of non-equilibrium carriers due to any type of carrier recombination processes in the bulk and using the value R=85% for the reflection coefficient from the Si-metal surface. 
   Among the limitations of the disclosed structure is a small opening (photo sensitive) area. However, many of the fast photodiodes available on the market, especially those for the telecommunication applications, also have by design a small sensitive area of ca. 50 um×50 um or even smaller. Use of lenses, especially micro-lenses, allows focusing the optical beam into a small spot, thus improving the sensitivity of the devices. 
   The design allows building a photodiode with a rectangular-shaped opening area (a×b), with practically no restriction to the size b of a longer side. Using a cylindrical lens, one can sharp focus the optical beam into a narrow strip, relaxing the alignment requirements along one of the two coordinates. 
   The design allows building  1 D arrays with very small gaps between the elements.  FIG. 7  illustrates a single element, which can be reproduced to form an array. This Figure shows the added bond pad  210  attached to the anode (cathode) contact  150  and utilizes a backside wafer contact for the cathode (anode) connection.  FIG. 8  shows a grouping of 5 elements to form a small linear array. These have a common backside contact for the cathode (anode). 
   In the embodiment shown in  FIGS. 5 and 6 , the cathode (anode)  180  covers bottom and sides of the active volume  170 . This layer, which is more heavily doped than the active volume, need not cover the sides, but as a minimum, should cover most if not all the bottom of the active volume  170 . However, since it is necessary to make electrical contact from the cathode (anode)  180  to metalization  130  if both anode and cathode contacts are to be accessible from the same side of the photodiode, a convenient way to do this is to at least have the cathode (anode)  180  also cover at least part of one side of the active volume  170  for continuity purposes. Also, mask layer  190  is shown not contacting metal contact  150 . However, even the mask layer is a metal layer, formed by patterning a metal layer to simultaneously form the electrical contacts and the mask layer, the metal mask layer may electrically contact the metal contact  150 , as the metal contact  150  will be at the same voltage as the anode (cathode)  160  under the mask layer and therefore not significantly increase the resulting capacitance of the anode (cathode)  160 . 
   A process to produce these devices or arrays would start with a &lt;100&gt; substrate of about 400 Ohm-cm. An N-type (P-type) substrate would be used for a top Anode (Cathode). Different resistivity values could be used as long as the active volume  170  in  FIG. 5  is essentially fully depleted when biased. An oxide is deposited or grown on the surface, and patterned to expose those areas, which will be removed to form the angled mirror surfaces. An isotropic etch such as KOH is used to form these mirrors, after which the oxide layer is removed. 
   At this point the N-type cathode (P-type anode)  180  is applied by an ion implant technique. It is also possible to use a standard furnace deposition for this step. An oxide layer  120  is then formed either by deposition or thermal growth, and the substrate material  110  is applied. The preferred embodiment uses polysilicon for this layer with a thickness of 30 um to 100 um although thicker layers may be used. The wafer is then inverted and the previous “bottom” is polished off exposing the new top surface of the device oriented as shown in  FIG. 5 . 
   Using a mask, the P-type anode (N-type cathode)  160  is applied by ion implantation, and top oxide layer  120  is grown or deposited. If grown, the furnace cycle will act to activate the implanted impurities. If deposited, a rapid thermal processing step will be used for activation. It is understood that normal furnace operations could be substituted for the ion implantation and activation steps. The contact openings are formed by normal photolithography and etch techniques, and metal is deposited and patterned to form the contacts and bond pads  130  and  150 . This same metal layer can be used to form the optional light-masking layer  190  if desired. 
   While certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.