Abstract:
A light sensor having a chemically resistant and robust reflector stack is disclosed. The reflector stack is formed over a substrate, and includes an adhesion layer, a patterned reflector layer over the adhesion layer, and a smoothing layer over the patterned reflector layer. The patterned reflector layer has a substantially flat top surface. A conformal passivation layer covers the reflector stack. An absorbing layer is situated above the reflector stack and separated from the reflector stack. The absorbing layer is supported by vias over the substrate. The absorbing layer is connected to at least one resistor, where a resistance of the at least one resistor varies in response to light absorbed by the absorbing layer. The vias are disposed on via landing pads on the substrate.

Description:
BACKGROUND 
       [0001]    The present application claims the benefit of and priority to a provisional patent application entitled “Light Sensor with Chemically Resistant and Robust Reflector Stack,” Ser. No. 62/045,706 filed on Sep. 4, 2014. The disclosure in this provisional application is hereby incorporated fully by reference into the present application. 
         [0002]    Light sensors, such as infrared sensors, can operate on the principle that electrical resistance of an absorbing layer changes in response to radiation reflected from a reflector underlying the absorbing layer. For example, when the absorbing layer gets heated by radiation reflected from the reflector, a change in electrical resistance can be detected by a readout integrated circuit. The sensitivity of the light sensor may depend on many factors, such as the planarity of the reflector. For example, a substantially flat reflector surface can greatly improve a signal to noise ratio to ensure the sensitivity of the light sensor. 
         [0003]    Since the reflector of the light sensor is an underlying layer below the absorbing layer, the reflector is formed, for example, on a substrate before other features of the light sensor are formed. As a result, the reflector is subject to harsh chemical and temperature environments during downstream processing actions, such as polishing, high power asking, and etching. Thus, the reflective properties of the reflector in the light sensor can be greatly impacted by these downstream processing actions, resulting in a decreased sensitivity of the light sensor. 
         [0004]    Thus, there is a need in the art for robust and chemically resistant reflector structures to enhance structural and functional performance and stabilities of the light sensors. 
       SUMMARY 
       [0005]    The present disclosure is directed to a light sensor with chemically resistant and robust reflector stack, substantially as shown in and/or described in connection with at least one of the figures, and as set forth in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a flowchart illustrating a method for fabricating a light sensor with a chemically resistant and robust reflector stack according to one implementation of the present application. 
           [0007]      FIG. 2A  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with an initial action in the flowchart of  FIG. 1  according to one implementation of the present application. 
           [0008]      FIG. 2B  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with an intermediate action in the flowchart of  FIG. 1  according to one implementation of the present application. 
           [0009]      FIG. 2C  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with an intermediate action in the flowchart of  FIG. 1  according to one implementation of the present application. 
           [0010]      FIG. 2D  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with an intermediate action in the flowchart of  FIG. 1  according to one implementation of the present application. 
           [0011]      FIG. 2E  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with an intermediate action in the flowchart of  FIG. 1  according to one implementation of the present application. 
           [0012]      FIG. 2F  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with an intermediate action in the flowchart of  FIG. 1  according to one implementation of the present application. 
           [0013]      FIG. 2G  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with an intermediate action in the flowchart of  FIG. 1  according to one implementation of the present application. 
           [0014]      FIG. 2H  illustrates a cross-sectional view of a portion of a light sensor with a chemically resistant and robust reflector stack processed in accordance with a final action in the flowchart of  FIG. 1  according to one implementation of the present application. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The following description contains specific information pertaining to implementations in the present disclosure. The drawings in the present application and their accompanying detailed description are directed to merely exemplary implementations. Unless noted otherwise, like or corresponding elements among the figures may be indicated by like or corresponding reference numerals. Moreover, the drawings and illustrations in the present application are generally not to scale, and are not intended to correspond to actual relative dimensions. 
         [0016]      FIG. 1  shows a flowchart illustrating an exemplary method of forming a light sensor according to an implementation of the present inventive concepts. Certain details and features have been left out of flowchart  100  that are apparent to a person of ordinary skill in the art. For example, an action may consist of one or more subactions or may involve specialized equipment or materials, as known in the art. Actions  180 ,  182 ,  184 ,  186 ,  188 ,  190 ,  192  and  194  indicated in flowchart  100  are sufficient to describe one implementation of the present inventive concepts, other implementations of the present inventive concepts may utilize actions different from those shown in flowchart  100 . Moreover, structures  280 ,  282 ,  284 ,  286 ,  288 ,  290 ,  292  and  294  in  FIGS. 2A ,  2 B,  2 C,  2 D,  2 E,  2 F,  2 G and  2 H illustrate the results of performing actions  180 ,  182 ,  184 ,  186 ,  188 ,  190 ,  192  and  194  of flowchart  100 , respectively. For example, structure  280  is an exemplary structure of a portion of a light sensor after processing action  180 , structure  282  is an exemplary structure of a portion of a light sensor after the processing of action  182 , structure  284  is an exemplary structure of a portion of a light sensor after the processing of action  184 , and so forth. 
         [0017]    Referring to action  180  in  FIG. 1  and structure  280  in  FIG. 2A , action  180  of flowchart  100  includes forming an adhesion layer over a substrate. As illustrated in  FIG. 2A , structure  280  includes adhesion layer  204  formed over substrate  202 . Substrate  202  can include any suitable material to support a light sensor thereon. In one implementation, substrate  202  may be a semiconductor substrate, having silicon, silicon-on-insulator (SOD, silicon-on-sapphire (SOS), silicon germanium, an epitaxial layer of silicon formed on a silicon substrate, or the like. In another implementation, substrate  202  may be a dielectric layer, such as an interlayer dielectric situated over an underlying semiconductor substrate. In yet another implementation, substrate  202  may be a dielectric substrate. Substrate  202  may include a readout integrated circuit (not explicitly shown in  FIG. 2A ) formed on or within the substrate. 
         [0018]    As illustrated in  FIG. 2A , adhesion layer  204  is formed over substrate  202 , and may include, for example, Titanium (Ti), Titanium Nitride (TiN), or a combination of both materials. Adhesion layer  204  is configured to promote adhesion between substrate  202  and a reflector metal layer, which is to be deposited over adhesion layer  204  in the subsequent action. Also, adhesion layer  204  can set up repeatable grain structures having small grain sizes for receiving reflector metal layer  206  to improve planarity at the interface between the two layers. In addition, the implementation of adhesion layer  204  may result in a smoother top surface of the reflector metal layer to be formed thereon. In the present implementation, adhesion layer  204  has a thickness between 50-250 Å. In other implementations, adhesion layer  204  may be greater than or less than 50-250 Å to suit the specific needs of a particular application. 
         [0019]    Referring to action  182  in  FIG. 1  and structure  282  in  FIG. 2B , action  182  of flowchart  100  includes forming a reflector metal layer on an adhesion layer, and planarizing a top surface of the reflector metal layer, for example, by Chemical Mechanical Polishing (CMP). As illustrated in  FIG. 2B , structure  282  includes reflector metal layer  206  formed over adhesion layer  204 . Reflector metal layer  206  may be formed by depositing a reflective metal layer over adhesion layer  204 , and planarizing a top surface thereof by using, for example, CMP. In one implementation, the planarizing of the top surface of the reflective metal layer may be optional. The thickness and planarity of reflector metal layer  206  can be further modulated by an etch process, such as a blanket etch after the CMP. 
         [0020]    As illustrated in  FIG. 2B , reflector metal layer  206  has a top planar surface that is configured to provide substantially uniform reflection. In the present implementation, reflector metal layer  206  includes pure aluminum. In another implementation, reflector metal layer  206  may include aluminum with 0.5 weight percent copper. In other implementations, reflector metal layer  206  may include titanium or any other suitable reflective materials. Reflector metal layer  206  may have a thickness between 500-2500 Å. In the present implementation, reflector metal layer  206  has a thickness of 1000 Å. In other implementations, reflector metal layer  206  may be greater than or less than 1000 Å to suit the specific needs of a particular application. For example, the thickness of reflector metal layer  206  may be determined based on the specific wavelength of interest of the light sensor. 
         [0021]    Referring to action  184  in  FIG. 1  and structure  284  in  FIG. 2C , action  184  of flowchart  100  includes forming a smoothing layer on a reflector metal layer. As illustrated in  FIG. 2C , structure  284  includes smoothing layer  208  formed over reflector metal layer  206 . Smoothing layer  208  may be formed by depositing Titanium (Ti), Titanium Nitride (TiN), or a combination of both materials over reflector metal layer  206 , and planarizing a top surface of thereof by using, for example, CMP. The thickness and planarity of smoothing layer  208  can be also modulated by an etch process, such as a blanket etch after a CMP. Thus, the CMP and/or the etching process can provide manufacturing process margin as well as suitable conductive and reflective functionality for the reflector stack. 
         [0022]    In the present implementation, smoothing layer  208  is configured to set up repeatable grain structures having small grain sizes to smooth bumpy aluminum in reflector metal layer  206 , thereby improving planarity of reflector metal layer  206 . For example, smoothing layer  208  can keep reflector metal layer  206  from becoming bumpy or turning in to spherical shapes when the light sensor undergoes a high thermal budget processing action, such as an anneal process. In the present implementation, smoothing layer  208  may have a thickness between 50-300 Å. In other implementations, smoothing layer  208  may be greater than or less than 50-300 Å, and may include other suitable capping materials, such as cladding materials. 
         [0023]    Referring to action  186  in  FIG. 1  and structure  286  in  FIG. 2D , action  186  of flowchart  100  includes patterning an adhesion layer, an reflector metal layer and an smoothing layer to form a reflector stack and via landing pads. As illustrated in  FIG. 2D , structure  286  includes reflector stack  210 , via landing pads  212   a  and  212   b  formed on substrate  202 . Reflector stack  210 , via landing pads  212   a  and  212   b  are formed, for example, by masking and patterning smoothing layer  208 , reflector metal layer  206  and adhesion layer  204  in structure  284  in  FIG. 2D . Reflector stack  210  includes patterned adhesion layer  204 , patterned reflector metal layer  206 , and patterned smoothing layer  208 . 
         [0024]    As illustrated in  FIG. 2D , via landing pads  212   a  and  212   b  have substantially the same structure and are formed of substantially the same material as reflector stack  210 , since they are patterned from structure  284  in the same processing action, action  186 . In structure  286 , a top surface of smoothing layer  208  and the sidewalls of patterned adhesion layer  204 , reflector metal layer  206 , and smoothing layer  208  in reflector stack  210  and via landing pads  212   a  and  212   b  are exposed, thus susceptible to corrosion and vulnerable chemicals and/or harsh environment during subsequent processing actions. 
         [0025]    Referring to action  188  in  FIG. 1  and structure  288  in  FIG. 2E , action  188  of flowchart  100  includes forming a conformal passivation layer over the reflector stack and via landing pads. As illustrated in  FIG. 2E , structure  288  includes conformal passivation layer  214  formed over reflector stack  210  and via landing pads  212   a  and  212   b  on substrate  202 . As illustrated in  FIG. 2E , conformal passivation layer  214  is coated on the top surfaces and sidewalls of patterned adhesion layer  204 , reflector metal layer  206 , and smoothing layer  208  in reflector stack  210  and via landing pads  212   a  and  212   b.    
         [0026]    In the present implementation, conformal passivation layer  214  includes silicon oxide due to silicon oxide&#39;s transparent property to many wavelengths and for allowing reflection of light from reflector stack  210  substantially without interference and/or obstruction. In another implementation, conformal passivation layer  214  may include silicon oxi-nitride (SiONx), silicon nitride (SiNx), or any combination thereof. In other implementations, conformal passivation layer  214  may include other materials, such as chemical and/or etch resistant materials. 
         [0027]    As illustrated in  FIG. 2E , conformal passivation layer  214  seals the entire reflector stack  210 , including the top surface of smoothing layer  208  and the sidewalls of adhesion layer  204 , reflector metal layer  206 , and smoothing layer  208 . Conformal passivation layer  214  can protect reflector stack  210 , especially, reflector metal layer  206 , against corrosion. Also, as discussed below, conformal passivation layer  214  can provide a barrier for reflector stack  210  against chemicals and/or harsh environment during subsequent processing actions, and provide structural and functional stabilities for the lifetime of the light sensor. In the present implementation, conformal passivation layer  214  has a thickness between 100-300 Å. In other implementations, conformal passivation layer  214  may be greater than or less than 100-300 Å to suit the specific needs of a particular application without compromising the reflective properties of reflector stack  210 . 
         [0028]    Referring to action  190  in  FIG. 1  and structure  290  in  FIG. 2F , action  190  of flowchart  100  includes forming a sacrificial layer over a conformal passivation layer. As illustrated in  FIG. 2F , structure  290  includes sacrificial layer  216  formed over conformal passivation layer  214 . In the present implementation, sacrificial layer  216  includes a polymer layer. For example, exemplary polymers that can be used to form sacrificial layer  216  may include, but not limited to, polyimides, polyamides (e.g., HD-2610), SU-8 photoresist, spin-on dielectrics (SOD), long chain polymers up to 10 microns. In the present implementation, sacrificial layer  216  has a thickness of about 1.5-2.5 microns. In other implementations, sacrificial layer  216  may have a thickness greater than or less than 1.5-2.5 microns to suit the specific needs of a particular application. 
         [0029]    Referring to action  192  in  FIG. 1  and structure  292  in  FIG. 2G , action  192  of flowchart  100  includes forming an absorbing layer and resistors over a sacrificial layer, and vias in a sacrificial layer. As illustrated in  FIG. 2G , structure  292  includes absorbing layer  222 , resistors  224   a  and  224   b,  and vias  218   a  and  218   b  formed on sacrificial layer  216 . As illustrated in  FIG. 2G , absorbing layer  222  and resistors  224   a  and  224   b  are formed on sacrificial layer  216 . Vias  218   a  and  218   b  extend through sacrificial layer  216  and conformal passivation layer  214  to make electrical and mechanical contact with via landing pads  212   a  and  212   b,  respectively. Absorbing layer  222  may include materials, such as amorphous silicon or vanadium oxide, to detect reflected radiation from reflector stack  210 . Resistors  224   a  and  224   b  may include metallic alloy, polysilicon, or other suitable resistive materials. Vias  218   a  and  218   b  may include tungsten (W), titanium (Ti), or any other suitable electrically conductive metallic material. As illustrated in  FIG. 2G , vias  218   a  and  218   b  are coated with dielectric liners  220   a  and  220   b,  respectively. Dielectric liners  220   a  and  220   b  may include tetraethylorthosilicate (TEOS) or oxide material, and can provide a rigid mechanical structure for vias  218   a  and  218   b,  such that vias  218   a  and  218   b  can stand on their own and provide support for absorbing layer  222 , for example. 
         [0030]    Referring to action  194  in  FIG. 1  and structure  294  in  FIG. 2H , action  194  of flowchart  100  includes removing a sacrificial layer to form a void separating an absorbing layer and a reflector stack. As illustrated in  FIG. 2H , structure  294  includes a light sensor, such as an infrared sensor, formed on substrate  202  after sacrificial layer  216  is removed from structure  292  in  FIG. 2G , for example. 
         [0031]    The removal of sacrificial layer  216  can be achieved by, for example, using oxygen plasmas or various other removal processes known in the art. In addition, chlorine rich and/or fluorine rich chemistry, such as chlorinated gases and/or fluorinated gases, can be used to during the removal of sacrificial layer  216 . During the removal process, for example, using oxygen plasma, oxygen plasma may attack metals (e.g., oxide metal surfaces), which would lead to corrosion and peeling of reflector stack  210 , if conformal passivation layer  214  were not present. Chlorine or fluorine containing gases can also attack metals to cause a reflecting surface to be uneven and/or non-planar. Thus, the presence of conformal passivation layer  214  provides a barrier for reflector stack  210  and via landing pads  212   a  and  212   b  against chemicals and/or harsh environment during the removal of sacrificial layer  216  and prevents corrosion and chemical attacks during the removal of sacrificial layer  216  and any other subsequent processing actions, thereby providing structural and functional stabilities for the lifetime of the light sensor in structure  294 , for example. 
         [0032]    As illustrated in  FIG. 2H , structure  294  includes a light sensor, such as an infrared sensor. In other implementations, structure  294  may include a microbolometer device or other types of optical sensors. Structure  294  includes reflector stack  210  underneath absorbing layer  222 . Reflector stack  210  includes patterned adhesion layer  204 , patterned reflector metal layer  206  and patterned smoothing layer  208 . Adhesion layer  204  is situated on substrate  202  to promote adhesion between substrate  202  and reflector metal layer  206 . Adhesion layer  204  is also configured to set up repeatable grain structures having small grain sizes for receiving reflector metal layer  206  to improve planarity at the interface between the two layers. In addition, the implementation of adhesion layer  204  results in a substantially flat top surface of reflector metal layer  206 . 
         [0033]    As illustrated in  FIG. 2H , reflector metal layer  206  is situated over adhesion layer  204 . Reflector metal layer  206  has a substantially flat top surface that can provide substantially uniform reflection. Smoothing layer  208  is situated over reflector metal layer  206 . Smoothing layer  208  is configured to keep reflector metal layer  206  from becoming bumpy or turning in to spherical shapes when structure  294  undergoes a high thermal budget processing action, such as an anneal process. Thus, adhesion layer  204 , reflector metal layer  206  and smoothing layer  208  together form reflector stack  210  on substrate  202 . 
         [0034]    As illustrated in  FIG. 2H , conformal passivation layer  214  is formed over reflector stack  210  and via landing pads  212   a  and  212   b.  Conformal passivation layer  214  seals the entire reflector stack  210 , including the top surface of smoothing layer  208  and the sidewalls of adhesion layer  204 , reflector metal layer  206 , and smoothing layer  208 , which would otherwise be exposed and susceptible to corrosion and vulnerable to chemicals and/or harsh environment during subsequent processing action. Thus, the presence of conformal passivation layer  214  protects the otherwise exposed surfaces. In addition, conformal passivation layer  214  can provide a barrier for reflector stack  210  against chemicals and/or harsh environment during subsequent processing actions, and provide structural and functional stabilities for the lifetime of the light sensor. Conformal passivation layer  214  can also provide protection for via landing pads  212   a  and  212   b , which may have substantially the same structure and are formed of substantially the same materials as reflector stack  210 . 
         [0035]    As further illustrated in  FIG. 2H , absorbing layer  222  is connected to resistors  224   a  and  224   b,  and suspended above reflector stack  210  with the support provided by vias  218   a  and  218   b.  Absorbing layer  222  is separated from reflector stack  210  by void  226 . Vias  218   a  and  218   b  are coated with dielectric liners  220   a  and  220   b,  respectively. Vias  218   a  and  218   b  are electrically and mechanically connected to via landing pads  212   a  and  212   b,  respectively, where signals from the light sensor can be transmitted to a readout integrated circuit (not explicitly shown in  FIG. 2H ) on or within substrate  202  through vias  218   a  and  218   b.  In the present implementation, structure  294  can be packaged in a vacuum. In other implementation, structure  294  can be packaged in air. 
         [0036]    In one implementation, the removal of sacrificial layer  216  from structure  292  in  FIG. 2G  is optional, such that the light sensor may include sacrificial layer  216  between reflector stack  210  and absorbing layer  222 , where sacrificial layer may be selected from substantially transparent material to any desirable wavelengths for allowing reflection of light from reflector stack  210  to absorbing layer  222  substantially without interference and/or obstruction. 
         [0037]    Thus, implementations of the present application utilize a reflector metal layer capped by a smoothing layer to modulate the thickness and planarity of the reflector stack, which can provide manufacturing process margin as well as suitable conductive and reflective functionality. A chemically resistant layer atop the reflector stack enables more aggressive and robust BEOL (back end of line) processing. Implementations of the present application can utilize standard CMOS process modules which are leveraged in such a way as to manufacture a module which is highly beneficial to one or more light sensors, such as infrared sensors and MEMS microbolometer devices. Implementations of the present application lend manufacturing paths to eliminating non-planarity in contrast to other processes incorporating non-planarity as part of their processing. 
         [0038]    From the above description it is manifest that various techniques can be used for implementing the concepts described in the present application without departing from the scope of those concepts. Moreover, while the concepts have been described with specific reference to certain implementations, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the scope of those concepts. As such, the described implementations are to be considered in all respects as illustrative and not restrictive. It should also be understood that the present application is not limited to the particular implementations described above, but many rearrangements, modifications, and substitutions are possible without departing from the scope of the present disclosure.