Abstract:
Methods for injecting charge include providing a target comprising a first layer on a second layer, coupling a conductive base to the second layer, and providing a medium which is in contact with at least a portion of the first layer. An electrode is positioned to face and is spaced from the first layer and is at least partially in contact with the medium. An electric field is provided across the first and second layers to inject charge to an interface between the first layer and the second layer.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present invention claims the benefit of U.S. Provisional Patent Application Ser. No. 60/498,891, filed Aug. 29, 2003, which is hereby incorporated by reference in its entirety. The subject invention was made with government support (Infotonics Technology Center (DOE)) Award No. DEFG02-02ER63410.A100. The U.S. Government may have certain rights. 

   FIELD OF THE INVENTION 
   The present invention generally relates to charge injection and, more particularly, to a method for distributed charge injection and a system thereof. 
   BACKGROUND 
   Embedded electronic charge technology is utilized in a variety of different types of MEMS devices and applications, such as those disclosed in U.S. Pat. Nos.: 6,597,560; 6,638,627; 6,688,179; 6,717,488; 6,750,590; and 6,773,488 and in U.S. Patent Application Publication Nos.: 2002/0131228; 2002/0182091; 2003/0079543; 2003/0201784; and 2004/0023236 by way of example. Embedded electronic charge technology is also extendable to a variety of applications in the macroscopic realm, such as heel strike power generation or for electrical generation from local environmental sources, such as the wind or waves. 
   These devices with embedded electronic charge use charge injection to trap charge at an interface of dissimilar insulators. Typically, the charge injection techniques used to embed the charge are high field injection or ballistic injection. High field injection requires a conducting material be placed on each side of the material into which charge is to be injected. The conducting materials on each side must be in substantial alignment with each other and a high voltage is applied across the conducting materials. Ballistic injection requires a ballistic charge source, such as an electron gun, a vacuum chamber, and a device to control energy, dose, and spatial coordinates of the ballistic charge source. 
   Unfortunately, it is difficult to inject electronic charge over an arbitrarily large surface and likewise to do so as a single step. If high field injection with conducting electrodes is used, a single defect, such as a short, in the material being injected will preclude charge injection. If ballistic injection is used, a very large and hence expensive vacuum chamber is required. 
   SUMMARY OF THE INVENTION 
   A method for injecting charge in accordance with embodiments of the present invention includes providing a target comprising a first layer on a second layer, coupling a conductive base to the second layer, and providing a medium which is in contact with at least a portion of the first layer. An electrode is positioned to face and is spaced from the first layer and is at least partially in contact with the medium. An electric field is provided across the first and second layers to inject charge to an interface between the first layer and the second layer. 
   A system for injecting charge in accordance with embodiments of the present invention includes a target comprising a first layer on a second layer, a conductive base to the second layer, a medium which is in contact with at least a portion of the first layer, an electrode and a electric field source, The electrode faces and is spaced from the first layer and is at least partially in contact with the medium. The electric field source provides an electric field across the first and second layers to inject charge to an interface between the first layer and the second layer. 
   The present invention provides a method and system for effectively establishing a desired charge density at trap sites at the interface of dissimilar insulators. The present invention is able to inject charge without the use of expensive systems and devices, such as vacuum chambers and ballistic electron sources. Furthermore, with the present invention there is no size constraint regarding the target into which charge is to be injected. Still further, the present invention can be used for charging other types of materials, such as a polymer electret film. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a distributed electrode injection system for injecting charge into a target in accordance with embodiments of the present invention; 
       FIG. 2  is a cross-sectional view of a distributed electrode injection system for injecting charge into predefined areas on a target in accordance with other embodiments of the present invention; 
       FIG. 3A  is a graph of capacitance-voltage characteristics before distributed electrode charge injection; and 
       FIG. 3B  is a graph of capacitance-voltage characteristics after distributed electrode charge injection. 
   

   DETAILED DESCRIPTION 
   A distributed electrode injection system  10 ( 1 ) in accordance with embodiments of the present invention is illustrated in  FIG. 1 . The distributed electrode injection system  10 ( 1 ) includes a conductive base  12 ( 1 ), a housing  14 ( 1 ) with a chamber  16 ( 1 ), an electrode  18 ( 1 ), and a power source  20 ( 1 ), although the system  10 ( 1 ) can include other types and numbers of components arranged in other manners, such as the distributed electrode injection system  10 ( 2 ) shown in  FIG. 2 . The present invention provides a number of advantages including a method and system for effectively establishing a desired charge density at trap sites at the interface of dissimilar insulators. 
   Referring more specifically to  FIG. 1 , a target  22  includes a layer of silicon dioxide (SiO 2 )  24  on a layer of silicon nitride (Si 3 N 4 )  26  on a layer of silicon dioxide (SiO 2 )  28  on a layer of silicon  30 , although the target can comprise other types and numbers of layers. The layer of silicon dioxide  24 , the layer of silicon nitride  26 , and the layer of silicon dioxide  28  each have a thickness of about 100 nm, although each of the layers  24 ,  26 , and  28  could have other thicknesses. An interface  25 ( 1 ) is formed between layers  24  and  26  and another interface  25 ( 2 ) is formed between layers  26  and  28 , although the target  22  can have other numbers of layers with other numbers of interfaces for storing injected charge. The layer of silicon  30  is placed on and coupled to the conductive base  12 ( 1 ), although the target  22  can be coupled to the conductive base  12 ( 1 ) in other manners. A variety of different types of conducting materials can be used for the conductive base  12 ( 1 ). 
   The housing  14 ( 1 ) is a hollow cylinder which defines the chamber  16 ( 1 ), although the housing  14 ( 1 ) could have other shapes. The chamber  16 ( 1 ) also has a cylindrical shape and has a pair of openings  32 ( 1 ) and  32 ( 2 ), although chamber  16 ( 1 ) could have other shapes with other numbers of openings. A seal  34  is positioned around an outer edge of the opening  32 ( 2 ) to the chamber  16 ( 1 ) in the housing  14 ( 1 ) and against the layer of silicon dioxide  24 , although the housing  14 ( 1 ) could be sealed against the target  22  in other manners. If electrode  18 ( 1 ) is near the target  22 , then sealing may not be required and housing  14 ( 1 ) can be directly on conductive base  12 ( 1 ). Housing  14 ( 1 ) is made of an insulating material, although other materials could be used. 
   A pair of securing brackets  36 ( 1 ) and  36 ( 2 ) each comprising a threaded rod  38 ( 1 ) or  38 ( 2 ), nuts  40 ( 1 ) and  40 ( 2 ) or  40 ( 3 ) and  40 ( 4 ) and a clamping plate  42 ( 1 ) or  42 ( 2 ) are used to secure the housing  14 ( 1 ), the target  22  and the conductive base  12 ( 1 ) together, although the housing  14 ( 1 ), the target  22  and/or the conductive base  12 ( 1 ) can be connected together in other manners. By way of example only, a single clamping plate with an opening could be used in place of the clamping plates  42 ( 1 ) and  42 ( 2 ). Each of the rods  38 ( 1 ) and  38 ( 2 ) is threaded at each end for engagement with one of the nuts  40 ( 1 )- 40 ( 4 ). One end of each of the rods  38 ( 1 ) and  38 ( 2 ) extends partially through openings in the conductive base  12 ( 1 ) and nuts  40 ( 1 ) and  40 ( 3 ) are respectively threaded on those ends of rods  38 ( 1 ) and  38 ( 2 ). Similarly, an opposing end of each of the rods  38 ( 1 ) and  38 ( 2 ) extends partially through openings in the clamping plates  42 ( 1 ) and  42 ( 2 ) and nuts  40 ( 2 ) and  40 ( 4 ) are respectively threaded on those ends of rods  38 ( 1 ) and  38 ( 2 ). When the housing  14 ( 1 ) and target  22  are placed between the conductive base  12 ( 1 ) and the clamping plates  42 ( 1 ) and  42 ( 2 ), the nuts  40 ( 1 )- 40 ( 4 ) are tightened on the threaded ends of the rods  38 ( 1 ) and  38 ( 2 ). Tightening the nuts  40 ( 1 )- 40 ( 4 ) secures the seal  34  around the opening  32 ( 2 ) to the chamber  16 ( 1 ) of the housing  14 ( 1 ) against the layer of silicon dioxide  24  of the target  22 . 
   The chamber  16 ( 1 ) which is defined by the wall of the housing  14 ( 1 ), the seal  34 , and the layer of silicon dioxide  24  of the target  22  is partially filled with a medium  44 . In these embodiments, the medium  44  is a fluid, such as water, with a low conductivity between about between about 1×10 −4  and 5×10 −8  siemens and a high permittivity above 80, although other types of mediums, such as a solid material or a solid in the form of a powder or other types of fluids, such as organic or mineral oil, can be used. For other mediums, the conductivity can have other ranges for conductivity, such as between about 1×10 −4  and 1×10 −10 . Additionally, the medium  44  can have other conductivity and permittivity ranges. 
   The electrode  18 ( 1 ) is positioned in the medium  44  in the chamber  16 ( 1 ) and is spaced from the target  22 . The spacing of the electrode  18 ( 1 ) from the target  22  depends on the type of medium  44  in the chamber  16 ( 1 ). For example, if the medium  44  is an extremely high resistivity liquid, such as 18×10 6  Ω-cm water, the electrode  18 ( 1 ) may be close to the target  22 , in this example about one cm to three cm apart. If the medium  44  is somewhat conductive with a conductivity of 1×10 −6  siemens for example, then the electrode  18 ( 1 ) can be further from the target  22 , in this example about three cm to ten cm apart, because most of the electric field appears across the target  22  into which charge is to be injected and stored. Other distances can be used, including on the order of as little as a few microns by way of example. 
   The power source  20 ( 1 ) is coupled to the electrode  18 ( 1 ) and to the conductive base  12 ( 1 ) and supplies voltage to generate the electric field, which is used to inject the charge into the interface  25 ( 1 ) between the layer of silicon dioxide  24  and the layer of silicon nitride  26  and into the interface  25 ( 2 ) between the layer of silicon nitride  26  and the layer of silicon dioxide  28 . A variety of different types of power sources can be used for the power source  20 ( 1 ) and the amount of voltage applied to generate the electric field can vary based on the particular application. 
   Referring to  FIG. 2 , a distributed electrode injection system  10 ( 2 ) in accordance with other embodiments of the present invention is illustrated. Elements in  FIG. 2  which correspond to those disclosed with reference to  FIG. 1  will have like reference numerals and will not be described again in detail. 
   A target  46  includes a layer of silicon dioxide (SiO 2 )  48  on a layer of silicon nitride (Si 3 N 4 )  50  on a layer of silicon dioxide (SiO 2 )  52  on a layer of silicon  54 , although the target can comprise other types and numbers of layers. For example, other types of layers which have a low permittivity with respect to the medium  44  can be used for the layer of silicon dioxide  48 . The layer of silicon dioxide  48  is etched to form an opening  55  which exposes a portion of the layer of silicon nitride  50 , although the target  46  could be etched in other manners or could have no etchings. In this example, the layer of silicon dioxide  48  has a thickness of about one micron, the layer of silicon nitride  50  has a thickness of about 100 nm, and the layer of silicon dioxide  52  has a thickness of about 100 nm, although each of the layers  48 ,  50 , and  52  could have other thicknesses. The layer of silicon  54  is placed on and coupled to the conductive base  12 ( 2 ), although the target  46  can be coupled to the conductive base  12 ( 2 ) in other manners. A variety of different types of conducting materials can be used for the conductive base  12 ( 2 ). 
   The housing  14 ( 2 ) is a hollow cylinder which defines the chamber  16 ( 2 ), although the housing  14 ( 2 ) could have other shapes. Housing  14 ( 2 ) is made of an insulating material, although other materials could be used. The chamber  16 ( 2 ) also has a cylindrical shape and has a pair of openings  56 ( 1 ) and  56 ( 2 ), although chamber  16 ( 2 ) could have other shapes with other numbers of openings. The conductive base  12 ( 2 ) is secured in the chamber  16 ( 2 ) of the housing  14 ( 2 ) adjacent the opening  56 ( 2 ). The conductive base  12 ( 2 ) seals the opening  56 ( 2 ) to form a container in the chamber  16 ( 2 ), although the opening  56 ( 2 ) to the chamber  16 ( 2 ) of the housing  14 ( 2 ) could be sealed in other manners. 
   The chamber  16 ( 2 ) which is defined by the wall of the housing  14 ( 2 ) and the conductive base  12 ( 2 ), is partially filled with the medium  44 . The medium  44  is described in greater detail earlier. The electrode  18 ( 2 ) is positioned in the medium  44  in the chamber  16 ( 2 ) and is spaced from the target  46 . The spacing of the electrode  18 ( 2 ) from the target  46  also depends on the type of medium  44  in the chamber  16 ( 2 ), on the power supply  12 ( 2 ) and applied voltage, and on the material properties and geometry of the target  46  as described earlier with respect to electrode  18 ( 1 ) and target  22 . 
   The power source  20 ( 2 ) is coupled to the electrode  18 ( 2 ) and to the conductive base  12 ( 2 ) and supplies power to generate the electric field which is used to inject the charge into the interface  58  between the layer of silicon nitride  50  and the layer of silicon dioxide  52  in the etched region  55 . A variety of different types of power sources can be used for the power source  20 ( 2 ). 
   A method for injecting charge in accordance with embodiments of the present invention will now be described with reference to  FIG. 1 . A surface of the layer of silicon  30  for the target  22  is placed on the conductive base  12 ( 1 ) which couples the layer of silicon  30  to the conductive base  12 ( 1 ). The seal  34  is positioned around an outer edge of the opening  32 ( 2 ) to the chamber  16 ( 1 ) in the housing  14 ( 1 ). The seal  34  around the outer edge of the opening  32 ( 2 ) to the chamber  16 ( 1 ) in the housing  14 ( 1 ) is placed against a surface of the layer of silicon dioxide  24 . 
   The clamping plates  42 ( 1 ) and  42 ( 2 ) are positioned around the other end of the housing  14 ( 1 ) adjacent the opening  32 ( 1 ) so that the housing  14 ( 1 ) and target  22  are between the conductive base  12 ( 1 ) and the clamping plates  42 ( 1 ) and  42 ( 2 ). The nuts  40 (l)- 40 ( 4 ) are tightened on the threaded ends of the rods  38 ( 1 ) and  38 ( 2 ) which brings the clamping plates  42 ( 1 ) and  42 ( 2 ) towards the conductive base  12 ( 1 ) and secures the seal  34  around the opening  32 ( 2 ) to the chamber  16 ( 1 ) of the housing  14 ( 1 ) against the layer of silicon dioxide  24  of the target  22 . 
   Next, the chamber  16 ( 1 ) is partially filled with the medium  44 . Since the medium  44 , in this example water with low ionic concentration, has sufficiently high resistance, local shorting paths, such as pinholes in the target  22 , only preclude charge injection in the immediate vicinity of the pinhole. This effect is insignificant since it is limited to only local defect areas. 
   The electrode  18 ( 1 ) is placed in the medium  44  and is spaced from the target  22 , although the electrode could be placed in the chamber  16 ( 1 ) before the medium  44  is introduced. The distance for the spacing of the electrode  18 ( 1 ) from the target  22  depends on the type of medium  44  in the chamber  16 ( 1 ), on the power supply  12 ( 2 ) and applied voltage, and on the material properties and geometry of the target  22  as explained earlier. 
   The power source  20 ( 1 ) is coupled to the electrode  18 ( 1 ) and to the conductive base  12 ( 1 ) and applies a bias across the electrode  18 ( 1 ) and the conductive base  12 ( 1 ) which generates an electric field which is used to inject the charge into the interface  25 ( 1 ) between the layer of silicon dioxide  24  and the layer of silicon nitride  26  and into the interface  25 ( 2 ) between the layer of silicon nitride  26  and the layer of silicon dioxide  28 . The system  10 ( 1 ) is analogous to a voltage divider. Since the effective resistivity of the target  22  is large with respect to the resistivity of the medium  44 , in this example low conductivity water, most of the potential is dropped across the target  22 . With an appropriate applied bias level by the power source  12 ( 1 ) which provides an electric field equal to or greater than the electric field necessary for charge injection, electrons are injected from the layer of silicon  30  into the target  22 . Electrons are trapped at trap sites at interfaces  25 ( 1 ) and  25 ( 2 ) and remain there. Charge levels exceeding 1×10 13  e − /cm 2  have been experimentally achieved using this system  10 ( 1 ) and method. The charge trapped at the interfaces  25 ( 1 ) and  25 ( 2 ) is monopole charge. 
   For situations where the positive electrode  18 ( 1 ) is relatively close to the region subjected to charge injection, in this example layer  24 , using a voltage source  12 ( 1 ) with sufficiently high current capability, an edge seal  34  may not be required. The low conductivity fluid between about 1×10 −4  and 5×10 −8  siemens for the medium  44  will maintain the field virtually across the entire target  22 . 
   To demonstrate the effectiveness of this method and the insensitivity to local defects, a target  22  was placed in the system  10 ( 1 ) and, instead of high resistivity water, high concentration salt water was used for the medium  44 . With the salt water being highly conductive, bubbles indicated short sites or pinholes and the applied voltage could not be increased to greater than a few volts due to the maximum current limitation of the power supply  12 ( 1 ). Hence no charge injection was detected. The system  10 ( 1 ) and the same target  22  were cleaned and placed in the chamber  16 ( 1 ) again, but this time with low ionic concentration water as the medium  44 . The voltage was sustainable and significant charge injection was determined by post charge injection electrical characterization. 
   A method for injecting charge in accordance with other embodiments of the present invention will now be described with reference to  FIG. 2 . The method described with respect to  FIG. 2  is the same as the one described above with reference to  FIG. 1 , except as described below. 
   The opening  55  is etched into a target  46  at the location in which charge is to be injected, although the target  46  could be etched in other locations or could have no etchings. A surface of the layer of silicon  54  is placed on the conductive base  12 ( 2 ) which couples the target  46  to the conductive base  12 ( 2 ). 
   The chamber  16 ( 2 ) is partially filled with the medium  44  and electrode  18 ( 2 ) is positioned in the medium  44  in the chamber  16 ( 2 ) and is spaced from the target  46 . The spacing of the electrode  18 ( 2 ) from the target  46  also depends on the type of medium  44  in the chamber  16 ( 2 ) as described earlier. 
   The power source  20 ( 2 ) is coupled to the electrode  18 ( 2 ) and to the conductive base  12 ( 2 ) and applies a bias across the electrode  18 ( 2 ) and the conductive base  12 ( 2 ). This bias generates the electric field, which is used to inject the charge into the interface  58  between the layer of silicon nitride  50  and the layer of silicon dioxide  52  in the etched region  55 . Since charge injection is exponentially dependent on field, only areas with a sufficient electric field will result in charge injection. In this example, the opening  55  which exposed a portion of the layer of silicon nitride  50  is the only area with a sufficient electric field for charge injection. The remaining portion of the layer of silicon nitride  50  is covered by the layer of silicon dioxide  48  which prevents a sufficient field from being generated and thus from any charge injection occurring in that area The charge, in this example electrons, is trapped at the interface  25  and is a monopole charge. Accordingly, the above-described method is effective for injecting charge in predefined areas, such as specific devices on an appropriate silicon wafer. 
   Referring to  FIGS. 3A and 3B , graphs of the capacitance-voltage (C-V) response for layer of silicon oxide on a layer of silicon nitride on and layer of silicon oxide on a lightly doped n type silicon substrate before and after distributed electronic charge injection are illustrated As can be seen, a significant change is observed (C min  to C max  transition voltage) indicating a high level of injected, and subsequently stored, charge. These capacitance-voltage tests were performed using liquid metal, in this case InGa, as top electrode. Therefore, the areas of the capacitors were not known and varied for testing before and after charge injection. The area is not necessary for this test. Only the shift in the transition from C min  to C max  is required to determine the stored charge density. 
   Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.