Patent Publication Number: US-7586167-B2

Title: Detecting plasmons using a metallurgical junction

Description:
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   A portion of the disclosure of this patent document contains material which is subject to copyright or mask work protection. The copyright or mask work owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright or mask work rights whatsoever. 
   RELATED APPLICATIONS 
   The present invention is related to the following co-pending U.S. patent applications, each which is commonly owned with the present application at the time of filing, and the entire contents of each of which are incorporated herein by reference:
         1. U.S. application Ser. No. 10/917,571, filed on Aug. 13, 2004, entitled “Patterning Thin Metal Film by Dry Reactive Ion Etching”   2. U.S. application Ser. No. 11/203,407, filed Aug. 15, 2005, entitled “Method of Patterning Ultra-Small Structures,”   3. U.S. application Ser. No. 11/243,476, filed Oct. 5, 2005, entitled, “Structure and Methods for Coupling Energy from an Electromagnetic Wave;”   4. U.S. application Ser. No. 11/243,477, filed Oct. 5, 2005, entitled, “Electron Beam Induced Resonance;”   5. U.S. application Ser. No. 11/238,991, filed Sep. 30, 2005, entitled, “Light Emitting Free-Electron Micro-Resonant Structure;”   6. U.S. application Ser. No. 11/302,471, filed Dec. 14, 2005, entitled, “Coupled Nano-Resonating Energy Emitting Structures;”   7. U.S. application Ser. No. 11/325,432, filed Jan. 5, 2006, entitled, “Resonant Structure-Based Display;”   8. U.S. application Ser. No. 11/325,448, filed Jan. 5, 2006, entitled, “Selectable Frequency Light Emitter;”   9. U.S. application Ser. No. 11/325,571, filed Jan. 5, 2006, entitled, “Switching Micro-Resonant Structures by Modulating a Beam of Charged Particles;” and   10. U.S. application Ser. No. 11/325,534, filed Jan. 5, 2006, entitled, “Switching Micro-Resonant Structures Using at Least One Director.”       

   FIELD OF THE INVENTION 
   This relates in general to detector devices and, more particularly, to detector devices having a metallurgical junction. 
   INTRODUCTION AND BACKGROUND 
   Coupling energy from electromagnetic radiation in the frequency range from about 0.1 terahertz (THz) (3000 microns) to about 7 petahertz (PHz) (0.4 nanometers), referred to as the terahertz portion of the electromagnetic spectrum, is finding use in numerous new applications. These applications include improved detection of concealed weapons and explosives, forensics, improved medical imaging, detection of biological materials, better characterization of semiconductors; and broadening the available bandwidth for wireless communications. 
   In solid materials the detection of electromagnetic radiation starts with absorption, which is the mechanism for transferring energy from an electromagnetic (EM) wave to an electron-hole pair. In particular, photoconductor semiconductor devices use the absorption mechanism on receiving the EM wave and transfer the received energy via electron-hole pairs by band-to-band transitions. In addition, extrinsic photoconductor devices use the absorption mechanism and operate having transitions across the forbidden-gap energy levels (S. M., Sze, “Semiconductor Devices Physics and Technology”, 2002, page 285). 
   Photodetectors include a range of semiconductor devices. These devices can include various types of photodiodes such as heterojunction, avalanche, P-I-N, and the like. The absorption coefficient is a property of a material and defines the extent to which the material absorbs energy in the form of electromagnetic radiation. Cut-off wavelength is the wavelength below which a material normally does not absorb electromagnetic radiation. Representative semiconductor materials such as Silicon(Si), Germanium (Ge) and Gallium Arsenide (GaAs) have cut-off wavelengths of about 1.1 microns, 1.9 microns and 0.87 microns, respectively. Hence, one particular semiconductor material normally cannot absorb energy in both the visible (i.e., about 0.39 microns to about 0.77 microns) and the infrared (i.e., about 0.77 microns to about 1 millimeter) portions of the electromagnetic spectrum. 
   In a given metal the electron density is normally substantially uniform. Variation or modulation of the electron density is referred to as a charge density wave. Plasmons are a form of the charge density waves. By one definition, the particle name for the charge density wave is a plasmon. A particular type of plasmon typically occurs at an interface between a metal and a dielectric, or between a semiconductor and a dielectric, and is referred to as a surface plasmon. Measurement of features on a surface at ultra-high sensitivity can employ the use of surface plasmons. For example, the technology for measuring a microbe or a virus has recently developed through the use of surface plasmon detection. 
   One method, called the minimum reflection method, for detecting surface plasmons includes directing an electromagnetic wave at an angle incident to a dielectric-metal or dielectric-semiconductor interface. Generally, the EM wave is reflected off the dielectric-metal interface. As the angle of the incident EM wave is varied, a particular angle is reached where the reflected EM wave is substantially zero. At this particular angle, energy of the incident EM wave is generally transferred to the surface plasmons or plasmons. Hence, the angle at which the reflected EM wave is minimum indicates the detection of plasmons. A so-called Kretschmann-Raether configuration applies the minimum reflection method (above) and provides easy access by employing a prism that contacts a metal or semiconductor layer. An electromagnetic wave passes through the prism and can reflect off the layer. An Otto arrangement disposes a prism a distance from an interface of the metal or semiconductor layer and detects plasmons again using the minimum reflection technique. This arrangement presents a disadvantage, because the interface is difficult to access with the detector. In yet another configuration, a corrugated surface or grating can be used to detect the minimum reflection of the EM wave. Another method for detecting plasmons collects an image of the reflected EM wave. The image can be processed using digital signal processing (DSP) to provide an angle of resonance within a few microns. This method is generally costly. In U.S. Pat. No. 5,792,667, plasmons are detected by measuring a temperature rise on the metal or semiconductor layer by using an ultra-thin-film thermometer. This method has the disadvantages of requiring ideal temperature control and precise calibration of the thermometer. 
   We describe a structure for receiving electromagnetic radiation, stimulating plasmons and generating a current on detecting the plasmons. This structure can be used as a plasmon detector. Optionally, the structure can be used to detect electromagnetic radiation over a broader range than any particular semiconductor detector. A plasmon source can be formed within a semiconductor device, such as a diode or transistor with a P-N junction. The plasmon source can include a transmission line, a microstructure, a micro-resonant structure having a cavity, a portion of metallization within a microcircuit, and the like. An electromagnetic wave can be received at the plasmon source, thereby stimulating plasmons. Fields are generated by the stimulated plasmons and coupled near the junction. The fields interact with a built-in electric field that typically occurs across the junction. This changes the band-gap and enables a current to couple through the structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description, given with respect to the attached drawings, may be better understood with reference to the non-limiting examples of the drawings, wherein like reference numbers designate like elements, and wherein: 
       FIGS. 1-5  are enlarged cross-sectional side views showing the process steps of forming a detector device; 
       FIG. 6  is an enlarged top-view of the detector device of  FIG. 5 ; 
       FIG. 7  is a diagram showing a band-gap of the detector device under bias; and 
       FIG. 8  is a diagram of the detector device showing a reduced band-gap under bias and receiving an EM wave. 
   

   DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS 
   In general, an electromagnetic wave having a frequency below the plasma frequency of a metal is reflected. Electrons within the metal shield the electric field of the electromagnetic wave. On the other hand, an electromagnetic wave having a frequency above the plasma frequency is coupled through the metal. Here, the electrons are unable to respond fast enough to shield the metal from the electromagnetic wave. Silver (Ag) is a material having a plasma frequency of about 714 terahertz (0.42 microns) or in ultraviolet portion of the electromagnetic spectrum. Thus, silver reflects electromagnetic radiation over the entire visible portion of the spectrum and below. Gold (Au) has a plasma frequency in the blue portion of the visible spectrum around 612 terahertz (0.49 microns), and copper (Cu) has a plasma frequency of about 566 terahertz (0.53 microns) in the green portion of the visible spectrum. Hence, silver, gold and copper are at least some metals that interact with electromagnetic radiation within the visible portion of the electromagnetic spectrum and below. 
   In the following, references to plasmons are not limiting and can include the more general case of charge density waves. 
   Generally, devices and methods for detecting electromagnetic radiation and plasmons are described. A junction, such as a metallurgical junction or PN junction, is formed between two regions, which have distinct conductivity types. The junction creates an inherent field. A plasmon source is positioned generally near the junction and receives an electromagnetic wave. The plasmon source generates plasmons having a field. An interaction occurs between the inherent field and the field from the plasmons. This causes the band-gap to diminish and reduces the width of the depletion region. Hence, a current is coupled through the device and indicates energy in the form of plasmons is detected. 
     FIG. 1  is an enlarged cross-sectional side-view showing a portion of a semiconductor component  100  near the beginning of fabrication. For example, a thin layer of a dielectric material can be formed on a major surface  4  of a substrate  2 . The dielectric layer, commonly referred to as a screen oxide  7 , can have a thickness of approximately 20 nanometers. The screen oxide  7  is a sacrificial layer that serves to collect dislodged particles resulting from subsequent processing employing an ion-implantation beam. The screen oxide  7  typically randomly scatters the beam to reduce channeling. A photoresist layer  3 , for example, can be patterned having an opening  11  which exposes a portion of the screen oxide  7 . Ion-implantation can be used to introduce a dopant into the substrate  2  to form a region  32 . The dopant can include any suitable impurity material such as arsenic, antimony, phosphorus, or the like. In another embodiment (not shown), a dopant can be introduced using a diffusion method, without a screen oxide. Various diffusion techniques can be used and are well known to those skilled in the art. The substrate  2  can include compound semiconductors, silicon-on-insulator, silicon-on-sapphire, silicon-on-spinel, silicon-on-nitride, silicon-on-oxide, and the like. The type of material for making the substrate  2  and the method of introducing a dopant are not limiting. 
     FIG. 2  shows a portion of the device  100  after a drive-in oxidation step. The dopant comprising region  32  is driven deeper into the substrate  2  using various diffusion techniques well known to those in the art. For example, a wet or dry oxidation process step can be used. Similar to the process shown for  FIG. 1 , another dopant of an opposite conductivity type is introduced into the substrate  2  to form a region  36  above the region  32 . This dopant can include any suitable impurity material such as boron, or the like. The photoresist  3  and screen oxide  7  are removed after the implantation steps. A final drive-in oxidation step can be used to redistribute the dopants of regions  32  and  36  and forms a dielectric layer  8 . Some depletion of the dopant in region  36  can occur in the process of growing the dielectric layer  8 . For this reason, the initial dose of the dopant in region  36  should allow for some loss in impurity concentration. After the final drive-in oxidation step, a junction  34  is established between the regions  32  and  36 . The formation of dielectric layer  8  can include using other techniques such as chemical vapor deposition (CVD) and plasma enhanced vapor deposition (PECVD) and is not limiting. 
     FIG. 3  shows a portion the device  100  after further processing. A photoresist (not shown) is patterned and formed on the dielectric layer  8 . Openings  15  are etched through the dielectric layer  8 . 
   Referring now to  FIG. 4 , a plasmon source  14  is formed. The plasmon source can include a transmission line, a microstructure, a micro-resonant structure having a cavity, a portion of metallization within a microcircuit, and is not limiting. The plasmon source  14  can be formed on either side of the junction  34 . The plasmon source  14  can include resonant, sub-wavelength and wavelength structures and can be sized to a multiple of the wavelength. The shape of the plasmon source  14  can include spherical, cubical, triangular-pyramidal and the like. In another embodiment (not shown), a plasmon source can extend over a range of distances from a junction and is not limiting. In yet another embodiment (not shown), a device, such as a transistor, can include a plasmon source near a junction and is not limiting as to the type of device. Portions of the region  36  exposed by the openings  15 , shown in  FIG. 3 , are etched away using, for example, an anisotropic reactive ion etch. As shown in  FIG. 4 , the plasmon source  14  comprises microstructures  17 A- 17 I and is formed above the junction  34 . The microstructures  17 A through  17 I are formed in the etched away portions of the region  36  and fill up to a surface  5  of the dielectric layer  8  using, e.g., techniques described above. The microstructures  17 A- 17 I can be made using materials that include at least gold, silver, copper, aluminum and the like. 
   A dielectric layer  9  is formed on the microstructures  17 A- 17 I and the dielectric layer  8 . The index of refraction of the dielectric layer  9  should be greater than the index of refraction of the dielectric layer  8 . Generally, the preferred thickness of the dielectric layer  9  can include a broad range of thicknesses. For example, the thickness of the dielectric layer  9  can include a range of thicknesses from about 750 Angstroms to about 3,000 Angstroms. The material for the dielectric layer  9  can include various transparent or translucent materials such as silicon oxide, silicon carbide, and the like. The dielectric layer  9  can be formed using techniques commonly known in the semiconductor industry. These techniques can include various forms of chemical vapor deposition (CVD) and plasma enhanced vapor deposition (PECVD). The material and technique for making the dielectric layer  9  can take a variety of forms and is not limiting. The dielectric layer  9  can be planarized using Chemical Mechanical Polishing (CMP). 
   A dielectric layer  10  is formed on the dielectric layer  9 . The index of refraction of the dielectric layer  10  should be less than the index of refraction of the dielectric layer  9 . The thickness of dielectric layer  10  can be similar to the thickness of the dielectric layer  9 . 
   A dielectric layer  12  is formed on the dielectric layer  10  to provide isolation and has an index of refraction less than the dielectric layer  9 . A photoresist layer (not shown) is patterned on the dielectric layer  12  to form openings  25  and  29  using techniques well known to those skilled in the art. For example, the opening  25  and  29  can be formed using an anisotropic reactive ion etch that stops on the surface  4 . An opening  27  can be similarly formed to stop on the dielectric layer  8 . 
   In  FIG. 5 , metallization  20  and  22  fills the respective openings  25  and  29  (see  FIG. 4 ). A dielectric material  16  is formed in the opening  27  (see  FIG. 4 ) using various techniques such as chemical vapor deposition (CVD) and plasma enhanced vapor deposition (PECVD). The dielectric material  16  should have an index of refraction similar to the index of refraction of the dielectric layer  9 . The metallization  20  and  22  electrically connects to regions  36  and  32 , respectively. The microstructures  17 A- 17 I are shown in  FIG. 5  respectively from left to right. 
   The dopant of region  32  provides excess carriers (e.g., electrons) that diffuse from the region  32  and combine with carriers (e.g., holes) in the region  36 . In addition, the dopant from region  36  provides excess carriers (e.g., holes) that travel or diffuse and combine with carriers (e.g., electrons) in the region  32 . This substantially reduces the population of carriers in a portion of the respective regions  32  and  36  in the vicinity of the junction  34 . Hence, a portion of the regions  32  and  36  within the dashed lines shown near junction  34  is commonly referred to as a depletion region  40 . Further, a portion of the carriers reaching their respective regions  32  and  36  across the junction  34  do not combine. These non-combining carriers are referred to as uncovered charges, which form space charge regions on each side of the junction  34 . The uncovered charges generate an inherent field  38 , which is directed from the uncovered charges (e.g., holes) across the junction  34 . A potential barrier or built-in voltage V bi  occurs across the junction  34  from the inherent field  38 , and for silicon is about 0.6 volt. 
   Electrodes  44  and  46  are formed on the respective metallization  20  and  22 . The electrode  46  is shown connected to ground. A voltage source (not shown) is connected to the electrode  44 . An applied voltage V B  from the voltage source is generally within a range from about zero volts to a voltage of less than the built-in voltage V bi  such that a current does not couple across the junction  34 . 
   A coupler  50  is for coupling an electromagnetic wave  24  (EM wave) to the microstructures  17 A- 17 I and includes the dielectric material  16  and a portion of the dielectric layers  8 ,  9 ,  10  and  12 . The dielectric layer  9  and the dielectric material  16  can serve as the core of the coupler  50 . The dielectric layers  8 ,  10  and  12  function as the coupler&#39;s  50  cladding. The EM wave  24 , shown traveling through the dielectric layer  9  of the coupler  50 , can be provided by an external source (not shown) coupled to an end  49  of the coupler  50 . A ferrule  48  as shown can be used to connect the external source to the coupler  50 . 
   At least a portion of the microstructures  17 A- 17 I is exposed to the core or the dielectric layer  9 , which carries the EM wave  24 . An interaction between the EM wave  24  and the microstructures  17 A- 17 I stimulate plasmons having fields  26 . The fields  26  can be intensified or concentrated at the corners  18  of the microstructures  17 A- 17 I. By changing the size, shape and material of the microstructures  17 A- 17 I, the EM wave  24  can interact with the microstructures  17 A- 17 I over a range of frequencies. 
   The fields  26  from the microstructures  17 A- 17 I can buck or oppose the inherent field  38  across the junction  34 . As a result, the band-gap is reduced, thereby allowing a current  28  to flow across the junction  34  and through the device  100 . Hence, the current  28  is generated on detecting the plasmons. This provides an alternative method for detecting plasmons. Further, as mentioned above, no particular semiconductor can absorb electromagnetic radiation across the entire visible portion of the electromagnetic spectrum and below. In contrast, silver, for example, interacts with electromagnetic radiation generally across the visible spectrum and below. Thus, the device  100  can be formed using the microstructures  17 A- 17 I made from silver, or a number of other materials that interact with electromagnetic radiation. The device  100  provides the advantage of using one structure to cover the entire visible spectrum and below. In contrast, semiconductor devices that use absorption to detect electromagnetic radiation cannot continuously function over the visible spectrum and below. Further, the response speed of semiconductor detectors is limited. Carriers within a semiconductor detector that are generated outside the depletion region are delayed, because they must travel to the junction. For this reason and to increase the absorption area, the width of the depletion region is typically increased. However, too wide a depletion region can also result in an increase of the travel-time of the carriers. In contrast, if the depletion region is biased too thin, capacitance will increase causing the response of the semiconductor detector to increase (S. M., Sze, “Semiconductor Devices Physics and Technology”, 2002, page 315). 
     FIG. 6  is an enlarged top-view of the device  100  shown in  FIG. 5 .  FIG. 6  illustrates a microstructure array  31  forming rows  13 ,  19 ,  21 ,  23  and  29 . In row  21 , the microstructures  17 A- 17 I are shown respectively from left to right. Electrodes  44  and  46  for respectively connecting to a voltage source and to ground are shown. 
     FIG. 7  is an energy diagram of a band-gap  30  between the top of the valence band EV and the bottom of the conduction band EC shown at an applied voltage of V B . Regions  32  and  36 , junction  34  and the boundaries of the depletion region  40  are overlaid on the energy diagram. The applied voltage V B  is shown connected to the electrode  44  in  FIG. 5  and has a magnitude between zero and the built-in potential V bi , (i.e., 0&lt;V B &lt;V bi ). The applied voltage V B  provides a potential energy difference (i.e., q (V bi −V B )) across the junction  34 . The energy difference across the junction  34  is the charge (q) (e.g., coulombs) multiplied by built-in voltage V bi  minus the applied voltage V B  (e.g., joule/coulomb). This decreases the total electrostatic potential across the junction  34  by the applied voltage V B . For example, a voltage of 0.3 volt is applied to the electrode  44  and opposes a built-in voltage V bi  of 0.6 resulting in a voltage of about 0.3 volt across the junction  34 . The band-gap  30  is a function of a number of variables including the net electrostatic potential across the junction  34 . The band-gap  30  can be increased or decreased by an applied voltage across the device  100 . Here, the band-gap  30  is reduced by the applied voltage V B , because the field from the applied voltage V B  opposes the inherent field  38  of the built-in voltage V bi . For current to begin to flow, a carrier (e.g., electron) needs sufficient energy to reach the lowest energy level of the conduction band E c . In other words, the carrier needs to overcome the energy difference (i.e., q (V bi −V B ) across the junction  34 . 
     FIG. 8  is an energy diagram of the device  100  illustrating a reduced band-gap  30  on receiving the EM wave  24 . The EM wave  24 , as mentioned, stimulates plasmons that generate the field  26 , which reduces the band-gap  30 . In other words, the field  26  opposes the inherent field  38 , thereby reducing the band-gap  30 . The field  26  introduces a plasmon voltage V P . The corresponding potential energy difference (i.e., q (V bi −V B −V P )) across the junction  34  is decreased by the plasmon voltage V P . Carriers (e.g., electrons) can now overcome the potential energy difference across the junction  34 . Thus, the current  28  flows across the junction  34  and through the device  100 . 
   Methods of making a device for detecting an electromagnetic wave as can be employed herein may use, e.g., the techniques described in related U.S. applications Ser. Nos. 10/917,571 and/or 11/203,407, each of which is described in greater detail above. 
   The devices described herein may also employ various similar or different example resonant structures like those described in one or more of the following, related applications, each of which was described in greater detail above: U.S. applications Ser. Nos. 11/243,476; 11/243,477; 11/238,991; 11/302,471; 11/325,432; 11/325,448; 11/325,571 and 11/325,534. 
   Thus are described devices and methods that detect plasmons. A metallurgical junction is formed and provides an inherent field across the junction. A built-in voltage results from the inherent field. A band-gap is established. The metallurgical junction can be biased between zero and the built-in voltage. A plasmon source is formed near the junction. An electromagnetic wave is received and channeled to expose the plasmon source, and plasmons are stimulated. A field occurs from the stimulated plasmons that oppose the inherent field. A net electrostatic potential across the junction is reduced. Thus, the band-gap is reduced, and a current is coupled across the junction and through the device on detecting the plasmons. 
   Although certain preferred embodiments and methods have been disclosed herein, it will be apparent from the foregoing disclosure to those skilled in the art that variations and modifications of such embodiments and methods may be made without departing from the spirit and scope of the invention. It is intended that the invention shall be limited only to the extent required by the appended claims and the rules and principles of applicable law.