Abstract:
A method of forming a semiconductor sensor in one embodiment includes providing a substrate, forming a reflective layer on the substrate, forming a sacrificial layer on the reflective layer, forming an absorber layer with a thickness of less than about 50 nm on the sacrificial layer, forming an absorber in the absorber layer integrally with at least one suspension leg, and removing the sacrificial layer.

Description:
FIELD 
       [0001]    This invention relates to semiconductor sensor devices and methods of fabricating such devices. 
       BACKGROUND 
       [0002]    Objects at any non-zero temperature radiate electromagnetic energy which can be described either as electromagnetic waves or photons, according to the laws known as Planck&#39;s law of radiation, the Stefan-Boltzmann Law, and Wien&#39;s displacement law. Wien&#39;s displacement law states that the wavelength at which an object radiates the most (λ max ) is inversely proportional to the temperature of the object as approximated by the following equation: 
         [0000]    
       
         
           
             
               
                 λ 
                 max 
               
                
               
                 ( 
                 
                   μ 
                    
                   
                       
                   
                    
                   m 
                 
                 ) 
               
             
             ≈ 
             
               3000 
               
                 T 
                  
                 
                   ( 
                   K 
                   ) 
                 
               
             
           
         
       
     
         [0003]    Hence for objects having a temperature close to room temperature, most of the emitted electromagnetic radiation lies within in the infrared region. Due to the presence of CO 2 , H 2 O, and other gasses and materials, the earth&#39;s atmosphere absorbs electromagnetic radiation having particular wavelengths. Measurements have shown, however, that there are “atmospheric windows” where the absorption is minimal. An example of such a “window” is the 8 μm-12 μm wavelength range. Another window occurs at the wavelength range of 3 μm-5 μm. Typically, objects having a temperature close to room temperature emit radiation close to 10 μm in wavelength. Therefore, electromagnetic radiation emitted by objects close to room temperature is only minimally absorbed by the earth&#39;s atmosphere. Accordingly, detection of the presence of objects which are either warmer or cooler than ambient room temperature is readily accomplished by using a detector capable of measuring electromagnetic radiation emitted by such objects. 
         [0004]    One commonly used application of electromagnetic radiation detectors is for automatically energizing garage door lights when a person or car approaches. Another application is thermal imaging. In thermal imaging, which may be used in night-vision systems for driver assistance, the electromagnetic radiation coming from a scene is focused onto an array of detectors. Thermal imaging is distinct from techniques which use photomultipliers to amplify any amount of existing faint visible light, or which use near infrared (˜1 μm wavelength) illumination and near-infrared cameras. 
         [0005]    Two types of electromagnetic radiation detectors are “photon detectors” and “thermal detectors”. Photon detectors detect incident photons by using the energy of said photons to excite charge carriers in a material. The excitation of the material is then detected electronically. Thermal detectors also detect photons. Thermal detectors, however, use the energy of said photons to increase the temperature of a component. By measuring the change in temperature, the intensity of the photons producing the change in temperature can be determined. 
         [0006]    In thermal detectors, the temperature change caused by incoming photons can be measured using temperature-dependant resistors (thermistors), the pyroelectric effect, the thermoelectric effect, gas expansion, and other approaches. One advantage of thermal detectors, particularly for long wavelength infrared detection, is that, unlike photon detectors, thermal detectors do not require cryogenic cooling in order to realize an acceptable level of performance. 
         [0007]    One type of thermal sensor is known as “bolometer.” Even though the etymology of the word “Bolometer” covers any device used to measure radiation, bolometers are generally understood to be to thermal detectors which rely on a thermistor to detect radiation in the long wavelength infrared window (8 μm -12 μm) or mid-wavelength infrared window (3 μm -5 μm). 
         [0008]    Because bolometers must first absorb incident electromagnetic radiation to induce a change in temperature, the efficiency of the absorber in a bolometer relates to the sensitivity and accuracy of the bolometer. Ideally, absorption as close to 100% of incident electromagnetic radiation is desired. In theory, a metal film having a sheet resistance (in Ohms per square) equal to the characteristic impedance of free space, laying over a dielectric or vacuum gap of optical thickness d will have an absorption coefficient of 100% for electromagnetic radiation of wavelength  4   d.  The following equation shows the expression of the characteristic impedance (Y) of free space: 
         [0000]    
       
         
           
             Y 
             = 
             
               
                 
                   μ 
                   0 
                 
                 
                   ɛ 
                   0 
                 
               
             
           
         
       
     
         [0000]    wherein ε 0  is the vacuum permittivity and μ 0  is the vacuum permeability. 
         [0009]    The numerical value of the characteristic impedance of free space is close to 377 Ohm. The optical length of the gap is defined as “nd”, where n is the index of refraction of the dielectric, air or vacuum. 
         [0010]    In the past, micro-electromechanical systems (MEMS) have proven to be effective solutions in various applications due to the sensitivity, spatial and temporal resolutions, and lower power requirements exhibited by MEMS devices. One such application is as a bolometer. Known bolometers use a supporting material which serves as an absorber and as a mechanical support. Typically, the support material is silicon nitride. A thermally sensitive film is formed on the absorber to be used as a thermistor. The absorber structure with the attached thermistor is anchored to a substrate through suspension legs having high thermal resistance in order for the incident electromagnetic radiation to produce a large increase of temperature on the sensor. 
         [0011]    The traditional technique used to micromachine suspended members involves the deposition of the material, such as by spin coating or polymer coating using a photoresist, over a “sacrificial” layer, which is to be eventually removed. The deposition of the thin-film metal or semiconductor can be done with a variety of techniques including low-pressure chemical vapor deposition (LPCVD), epitaxial growth, thermal oxidation, plasma-enhanced chemical vapor deposition (PECVD), sputtering, and evaporation. 
         [0012]    The know processes, however, have inherent limitations with respect to fabrication of bolometers. For example, in order to retain functionality, silicon wafers must not be exposed to temperatures higher than about 450° C. This temperature limitation eliminates several of the deposition techniques mentioned above. 
         [0013]    Additionally, it is very difficult to reliably fabricate a suspended thin-film metal using the traditional deposition techniques of sputtering, evaporation or PECVD due to problems of poor step coverage, thickness uniformity and control, and stress control. 
         [0014]    What is needed is an efficient and accurate bolometer. A further need exists for a bolometer that is easy and inexpensive to manufacture. 
       SUMMARY 
       [0015]    In accordance with one embodiment, there is provided a method of forming a semiconductor sensor including providing a substrate, forming a reflective layer on the substrate, forming a sacrificial layer on the reflective layer, forming an absorber layer with a thickness of less than about 50 nm on the sacrificial layer, forming an absorber in the absorber layer integrally with at least one suspension leg, and removing the sacrificial layer. 
         [0016]    In a further embodiment, a complementary metal oxide semiconductor (CMOS) sensor device includes a complementary metal oxide semiconductor (CMOS) substrate, at least one reflective component formed on the substrate, and at least one absorber spaced apart from the at least one reflective component, the at least one absorber formed by atomic layer deposition. 
         [0017]    In yet another embodiment, a complementary metal oxide semiconductor (CMOS) sensor device includes a complementary metal oxide semiconductor (CMOS) substrate, at least one reflective component formed on the substrate, and at least one absorber spaced apart from the at least one reflective component, the at least one absorber including a maximum thickness of less than 50 nm and exhibiting a good noise-equivalent temperature difference (NETD). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  depicts a top perspective view of a bolometer device with an absorber that provides the function of a thermistor in accordance with principles of the present invention; 
           [0019]      FIG. 2  depicts a side plan view of the bolometer of  FIG. 1 ; 
           [0020]      FIG. 3  depicts a top perspective view of another embodiment of a bolometer device with an absorber that provides the function of a thermistor in accordance with principles of the present invention; 
           [0021]      FIG. 4  depicts a side plan view of the bolometer of  FIG. 3 ; 
           [0022]      FIG. 5  depicts a cross-sectional view of a substrate, which in this embodiment is a complementary metal oxide semiconductor (CMOS), which may be used to form a device in accordance with principles of the present invention; 
           [0023]      FIG. 6  depicts a cross-sectional view of the substrate of  FIG. 5  with a sacrificial layer formed on the substrate and channels etched in the sacrificial layer; 
           [0024]      FIG. 7  depicts a cross-sectional view of the substrate of  FIG. 6  after conductive pillars have been formed in the etched channels; 
           [0025]      FIG. 8  depicts a cross-sectional view of the substrate of  FIG. 7  with an absorber layer deposited on the conductive pillars and on the portion of the sacrificial layer between the conductive pillars; 
           [0026]      FIG. 9  depicts a top plan view of the substrate of  FIG. 8  after trenches have been etched through the absorber layer to the sacrificial layer to form suspension legs and an absorber; 
           [0027]      FIG. 10  depicts a top perspective view of another embodiment of a bolometer device with an absorber that provides the function of a thermistor in accordance with principles of the present invention; 
           [0028]      FIG. 11  depicts a top plan view of the device of  FIG. 10 ; 
           [0029]      FIG. 12  depicts a cross-sectional view of a substrate, which in this embodiment is a complementary metal oxide semiconductor (CMOS), with a reflective layer formed on the substrate; 
           [0030]      FIG. 13  depicts a cross-sectional view of the substrate of  FIG. 12  with a sacrificial layer formed on the substrate and channels etched in the sacrificial layer; 
           [0031]      FIG. 14  depicts a cross-sectional view of the substrate of  FIG. 13  after base portions of conductive spring pillars have been formed in the etched channels; 
           [0032]      FIG. 15  depicts a cross-sectional view of the substrate of  FIG. 14  with another sacrificial layer formed on the substrate and channels etched in the sacrificial layer; 
           [0033]      FIG. 16  depicts a cross-sectional view of the substrate of  FIG. 15  after lower cross portions of conductive spring pillars have been formed in the etched channels and another sacrificial layer formed on the substrate and channels etched in the sacrificial layer; 
           [0034]      FIG. 17  depicts a cross-sectional view of the substrate of  FIG. 16  after center upright portions of conductive spring pillars have been formed in the etched channels; 
           [0035]      FIG. 18  depicts a cross-sectional view of the substrate of  FIG. 15  with another sacrificial layer formed on the substrate and channels etched in the sacrificial layer; 
           [0036]      FIG. 19  depicts a cross-sectional view of the substrate of  FIG. 18  after upper cross portions of conductive spring pillars have been formed in the etched channels and another sacrificial layer formed on the substrate and channels etched in the sacrificial layer; 
           [0037]      FIG. 20  depicts a cross-sectional view of the substrate of  FIG. 19  after upper upright portions of conductive spring pillars have been formed in the etched channels and an absorber layer deposited on the conductive spring pillars and on the portion of the sacrificial layer between the conductive pillars; and 
           [0038]      FIG. 21  depicts a cross-sectional view of the device of  FIG. 20  with the sacrificial layers removed. 
       
    
    
     DESCRIPTION 
       [0039]    For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and described in the following written specification. It is understood that no limitation to the scope of the invention is thereby intended. It is further understood that the present invention includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the invention as would normally occur to one skilled in the art to which this invention pertains. 
         [0040]      FIG. 1  depicts a perspective view of a semiconductor sensor  100  which in this embodiment is a bolometer. The sensor  100  may be formed on a complementary metal oxide semiconductor (CMOS) substrate or on another type of substrate. The sensor  100  includes a substrate  102 , a mirror  104  and an absorber  106 . The substrate  102 , which in this embodiment is a silicon wafer that may include one or more sensors  100 , includes the electronic circuitry used to access the output of the sensor  100 . 
         [0041]    The mirror  104  may be, for example, a metal reflector or a multilayer dielectric reflector. The absorber  106  is spaced apart from the mirror  104  by suspension legs  108  and  110 . In this embodiment, the gap between the mirror  104  and the absorber  106  is about 2.5 μm. The gap in this embodiment is selected to optimize absorption in the long-wavelength infrared region. 
         [0042]    The absorber  106 , in addition to absorbing energy from incident photons, is selected to provide a good noise-equivalent temperature difference (NETD). In order for the absorber  106  to have a good NETD, the material selected to form the absorber  106  should exhibit a high temperature coefficient of resistance while exhibiting low excess noise (1/f noise). Semiconductor materials such as vanadium oxide are common in micromachined bolometers due to their high temperature coefficient of resistance. While metals have a lower temperature coefficient of resistance than some semiconductor materials, such as vanadium oxide, metals typically have much lower excess noise than many semiconductor materials. 
         [0043]    Accordingly, in one embodiment the absorber  106  comprises metal. Titanium and Platinum are two metal which exhibit desired characteristics. Titanium, for example, exhibits a bulk resistivity of about 7*10 −7  Ohm. Using a bulk resistivity of 7*10 −7  Ohm, the thickness of the absorber  106  to match the impedance of free-space (377 Ohm/square) should be about 1.9 nm. The resistivity of materials formed to a thickness less than about 50 nm, however, can be several times higher than the bulk value. Accordingly, depending on process parameters, the thickness of the absorber  106 , if made from titanium, is preferably about 10 nm. Impurities can also be introduced into the absorber  106  during formation in order to tune the resistivity if needed. 
         [0044]    Consequently, the thickness of the absorber  106  in this embodiment is about 10 nm and the length of the absorber  106  from the suspension leg  108  to the suspension leg  110  is about 25 μm. This configuration provides a ratio between the thickness of the absorber  106  and the length of the absorber  106  in the order of 1/1000 and the ratio of the thickness of the absorber  106  to the gap width of about 1/100. 
         [0045]    The legs  108  and  110  provide mechanical support for the absorber  106  and are designed to have a high thermal resistivity. The total resistance for the sensor measured across the legs  108  and  110  and the absorber  106  is defined by the following equation: 
         [0000]        R= 2 R   s   +R   a    
         [0000]    where R s  is the resistance of each of the suspension legs  108  and  110  and R a  is the resistance of the thin-film absorber  106 . 
         [0046]    Upon impingement of the absorber  106  with electromagnetic radiation, the temperature of the absorber  106  increases by ΔT. Assuming that the temperature profile along the suspension legs  108  and  110  is linear, the average temperature increase of the suspension legs  108  and  110  is ΔT/2. The electrical resistance of the sensor upon incident radiation changes by an amount ΔR given by: 
         [0000]    
       
         
           
             
               Δ 
                
               
                   
               
                
               R 
             
             = 
             
               
                 2 
                  
                 α 
                  
                 
                     
                 
                  
                 
                   R 
                   s 
                 
                  
                 
                   
                     Δ 
                      
                     
                         
                     
                      
                     T 
                   
                   2 
                 
               
               + 
               
                 α 
                  
                 
                     
                 
                  
                 
                   R 
                   a 
                 
                  
                 Δ 
                  
                 
                     
                 
                  
                 T 
               
             
           
         
       
     
         [0000]    where α is the temperature coefficient of resistance of the thin film. Resolving the foregoing equation results in the following equation: 
         [0000]      Δ R=αΔT ( R   s   +R   a ) 
         [0047]    Because the legs  108  and  110  are designed to have a high thermal resistivity, the total electrical resistance of the sensor  100  is dominated by that of the suspension legs  108  and  110  (i.e. R s  is much greater than R a ) so that: 
         [0000]      ΔR≈αΔTR s    
         [0048]    Thus, when electromagnetic radiation (e.g. infrared light) reaches the sensor  100 , the electromagnetic radiation is absorbed within the thin-film metal of the absorber  106  with an efficiency depending on the resistivity of the absorber  106 , quality of the mirror  104 , gap height between the absorber  106  and the mirror  104 , and radiation wavelength. Upon absorbing the incident radiation, the absorber  106  undergoes an increase in temperature. This temperature increase, in turn, leads to either a decrease or increase of the resistivity of the absorber  106 . The absorber  106  is then electrically probed to measure the resistivity of, and thus indirectly measure the amount of incident electromagnetic radiation on, the absorber  106 . 
         [0049]    An alternative semiconductor sensor  120  is shown in  FIG. 3 . The semiconductor sensor  120  in this embodiment is also a bolometer which may be formed on a CMOS substrate. The sensor  120  includes a substrate  122 , a mirror  124  and an absorber  126 . The substrate  122 , which in this embodiment is a silicon wafer that may include one or more sensors  120 , includes the electronic circuitry used to access the output of the sensor  120 . 
         [0050]    The absorber  126  is supported by suspension legs  128  and  130 . The gap between the absorber  126  and the mirror  124  in this embodiment, however, is controlled by pillars  132  and  134 . The pillars  132  and  134 , in addition to establishing the gap between the absorber  126  and the mirror  124 , further provide electrical contact with the suspension legs  128  and  130 . Operation of the sensor  120  is substantially identical to the operation of the sensor  100 . 
         [0051]    Due to the typical resistivity of deposited metals and semiconductors, the suspended thin-film must have a thickness inferior to 50 nm. Features of the deposition technique known as atomic layer deposition is preferred over traditional micromachining techniques, e.g. sputtering and evaporation. One advantage of this device over the state of the art is its simplicity of fabrication. 
         [0052]    Fabrication of the sensor  120  begins with preparation of a substrate  140  which is shown in  FIG. 5  with a mirror  142  formed upon the substrate  140 . A sacrificial layer  144  of material is then deposited on the substrate  140 , and channels  146  and  148  are etched (see  FIG. 6 ). Conductive pillars  150  and  152  are then formed in the channels  146  and  148  as shown in  FIG. 7 . An absorber layer  154  is then formed on the conductive pillars  150  and  152  and over the sacrificial layer  144  between the conductive pillars  150  and  155  (see  FIG. 8 ). An “absorber layer” is a layer of material that exhibits efficient energy absorption from incident photons and good noise-equivalent temperature difference (NETD). As used herein, “good NETD” means that the material functions as a thermistor as well as an absorber. 
         [0053]    The absorber layer  154  is preferably formed by atomic layer deposition (ALD). ALD is used to deposit materials by exposing a substrate to several different precursors sequentially. A typical deposition cycle begins by exposing a substrate is to a precursor “A” which reacts with the substrate surface until saturation. This is referred to as a “self-terminating reaction.” Next, the substrate is exposed to a precursor “B” which reacts with the surface until saturation. The second self-terminating reaction reactivates the surface. Reactivation allows the precursor “A” to react with the surface. The deposition cycle results, ideally, in one atomic layer being formed, upon which, another layer may be formed. Accordingly, the final thickness of the absorber layer  154  is controlled by the number of cycles a substrate is exposed to. 
         [0054]    Typically, the precursors used in ALD include an organometallic precursor and an oxidizing agent such as water vapor or ozone. Atomic layer deposition has gained interest in recent years due to its ability to grow ultra-thin film at relatively low temperature with superior thickness control, uniformity and conformality. 
         [0055]    Once the absorber layer  154  is formed, the absorber layer  154  is etched to form suspension legs  156  and  158  and an absorber  160  ( FIG. 9 ). The sacrificial layer  144  is then removed to release the absorber  160 , resulting in a configuration as discussed with reference to  FIGS. 3 and 4 . 
         [0056]    An alternative semiconductor sensor  200  is shown in  FIG. 10 . The semiconductor sensor  200  in this embodiment is also a bolometer which may be formed on a CMOS substrate. The sensor  200  includes a substrate  202 , a mirror  204  and an absorber  206 . The substrate  202 , which in this embodiment is a silicon wafer that may include one or more sensors  200 , includes the electronic circuitry used to access the output of the sensor  200 . 
         [0057]    The absorber  206  is supported by suspension legs  208  and  210 . The gap between the absorber  206  and the mirror  204  in this embodiment is controlled by spring pillars  212  and  214 . The spring pillars  212  and  214 , in addition to establishing the gap between the absorber  206  and the mirror  204 , further provide electrical contact with the suspension legs  208  and  210 . Operation of the sensor  200  is substantially identical to the operation of the sensor  100 . 
         [0058]    Fabrication of the sensor  200  begins with preparation of a substrate  220  which is shown in  FIG. 12  with a mirror  222  formed upon the substrate  220 . A sacrificial layer  224  of material is then deposited on the substrate  220 , and channels  226  and  228  are etched (see  FIG. 13 ). The base portions  230  and  232  of the conductive pillars are then formed in the channels  226  and  228  as shown in  FIG. 14 . Another sacrificial layer  234  of material is then deposited on the sacrificial layer  224 , and channels  236  and  238  are etched (see  FIG. 15 ). After lower cross portions  240  and  242  of the conductive pillars are formed in the channels  236  and  238  (see  FIG. 16 ) a sacrificial layer  244  of material is then deposited on the sacrificial layer  224 , and channels  246  and  248  are etched. Middle uprights  250  and  252  are then formed in the channels  236  and  238  ( FIG. 17 ). 
         [0059]    Another sacrificial layer  254  of material is then deposited on the sacrificial layer  244 , and channels  256  and  258  are etched (see  FIG. 18 ). After upper cross portions  260  and  262  of the conductive pillars are formed in the channels  256  and  258  (see  FIG. 19 ) a sacrificial layer  264  of material is then deposited on the sacrificial layer  254 , and channels  266  and  268  are etched. Top uprights  270  and  272  are then formed in the channels  266  and  268  ( FIG. 20 ) and an absorber layer  274  is then formed on the top uprights  270  and  272  and over the sacrificial layer  264  between the conductive pillars Top uprights  270  and  272 . 
         [0060]    Once the absorber layer  274  is formed, the absorber layer  274  is etched to form suspension legs (see legs  208  and  210  of  FIG. 11 ) and the absorber (absorber  206  of  FIG. 11 ). The sacrificial layers  224 ,  234 ,  244 ,  254 , and  264 , which may be the same material, are then removed to release the absorber  206  ( FIG. 20 ), resulting in a configuration as discussed with reference to  FIGS. 10 and 11 . 
         [0061]    While the invention has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the invention are desired to be protected.