Abstract:
A Cd 1-x Zn x S film material, with a high value of thermal coefficient of resistance, in the range of 1.5% to 3.7%. The Cd 1-x Zn x S material has excellent characteristics for use in a microbolometer-type uncooled infrared sensor. The film material can be deposited on microbolometer membranes or any other wafer for different applications. The film material can be deposited using the MOCVD technique, thermal evaporation or a different technique to form the film material over the wafer. The Cd 1-x Zn x S properties can be modified controlling certain deposition parameters and different annealing techniques. The process is performed at temperature compatible with CMOS technology.

Description:
FIELD OF THE INVENTION 
   The present invention generally relates to high thermoelectric coefficient of resistance (TCR) material, and more particularly to a method for manufacturing high thermo-electric coefficient of resistance (TCR) material comprising Cd 1-x Zn x S film, which is used in bolometer-type uncooled infrared sensors (enabling a two-dimensional infrared imaging apparatus) and like devices using temperature variation of resistance. 
   BACKGROUND OF THE INVENTION 
   A microbolometer is the latest type of thermal imaging Focal Point Array (FPA), which consists of materials that measure heat by changing resistance at each pixel. A microbolometer is a type of infrared detector, which absorbs the infrared (IR) radiation and warms slightly. The electrical resistance across the bolometer changes as a function of temperature, which can be measured and calibrated. The most common microbolometer material is vanadium oxide (VOx). Amorphous silicon (a-Si) is another microbolometer material. The a-Si model has poor dynamic range and isothermal scene performance, which limits the current version(s) for many fire service applications. 
   It has been the practice in a bolometer type infrared sensor to use a titanium oxide film, a vanadium oxide film or a similar film. Examples are described in U.S. Pat. No. 5,286,976 issued to Barret E. Cole, U.S. Pat. No. 5,801,383 issued to Hideo Wada, an article contributed by Hubert Jerominek and others to Optical Engineering, v.32 (1993) n.9, pages 2092 to 2099 under the title of “Vanadium Oxide Films for Optical Switching and Detection,” an article contributed by D. P. Partlow and others and to Journal of Applied Physics, v.70 (1991) n.1, pages 443 to 452, under the title “Switchable Vanadium Oxide Films by a Sol-Gel Process.” Partlow discloses that vanadium oxide has thirteen distinct phases between vanadium dioxide and vanadium sesquioxide (rendering this material not stable enough). 
   Titanium oxide has a specific resistance as low as 0.01 Ohm.cm, however, the temperature coefficient is not so great in absolute value, being −0.2% per degree Celsius. On the other hand the specific resistance of vanadium dioxide is about 10 Ohm-cm when manufactured by sputtering. Its temperature coefficient is about −2%, when manufactured without additional temperature treatment. When additional thermal treatment is included, the TCR can be increased to 4% (see, for example, U.S. Pat. No. &#39;383 to Wada). However the treatment temperature is more than 350° C., which is not sufficiently compatible with the very large scale integrated circuit (VLSI) manufacturing process of the whole infrared sensor. Moreover, as described in U.S. Pat. No. &#39;383, to Wada, the vanadium dioxide is susceptible to a metal-semiconductor phase transition at about 70° C. This gives rise to a volume variation of vanadium dioxide and causes cracks and peel off to render a vanadium oxide film unreliable when it is used in the bolometer-type infrared sensor device, which is unavoidably subjected to temperature cycles passing through 70° C. 
   In U.S. Pat. No. 6,512,229 to Saski, et al, a process for preparing the crystal phase of V 2 O 3 , with x=1.5 in VO x  is disclosed. A TCR of about 2-4% is obtained after applying a heat treatment of 380° C. for 8 hours in hydrogen. However, the resistivity of these films is too low, about 10 −3 -10 −4  Ohm-cm. To tailor the resistivity to the needed values, about 10 −1  Ohm-cm, an additional oxidation heat treatment was applied. 
   Thus, it would be advantageous to provide a microbolometer film material with a high value of thermal coefficient of resistance, fabricated at temperatures compatible with CMOS technology. 
   SUMMARY OF THE INVENTION 
   Accordingly, It is a principal object of the present invention to provide a material with excellent characteristics for use in a bolometer-type infrared sensor. 
   It is another principal object of the present invention to provide a Cd 1-x Zn x S film, which has a high temperature coefficient of specific resistivity for use in bolometer-type sensors. 
   It is one other principal object of the present invention to provide a method for manufacturing microbolometer film material Cd 1-x Zn x S, with a thermal coefficient of resistance in the range of values from 1.5% to 3.7 %, for use at temperatures compatible with CMOS technology. 
   A method is disclosed for manufacturing a film material, Cd 1-x Zn x S, with a high value of thermal coefficient of resistance, in the range of 1.5% to 3.7%, wherein the film material can be deposited on a wafer. The method includes varying the composition of the deposited film material by adding a Zn component in accordance with Cd 1-x Zn x S. The method also includes establishing other deposition parameters including pressure, temperature, in order to determine resistivity of the deposited film material, growth rate and wafer type. Other steps include depositing the film material on the wafer and performing an annealing technique, such that the film material is fabricated at temperatures compatible with CMOS technology. 
   The film material of the present invention may be used in a predictive maintenance device, a fire safety device, security systems, night vision systems, living being detection sensors, satellite photography and for ultrasonic wave detection. 
   The film material can be deposited on microbolometer membranes or any other wafer for different applications. The film material can be deposited using the Metal-Organic Chemical Vapor Deposition (MOCVD) technique, thermal evaporation or a different technique to form the film material over the wafer. The Cd 1-x Zn x S properties can be modified controlling certain deposition parameters and different annealing techniques. The process is performed at low temperature compatible with CMOS technology. 
   Additional features and advantages of the invention will become apparent from the drawings and descriptions contained herein below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic illustration of the electrical contacts on the CdS film surface, constructed in accordance with the principles of the present invention; 
       FIG. 2  is a graphical representation of electrical resistance versus 1/T for appropriate TCR avg  of CdS on nitride and glass, in accordance with the principles of the present invention; 
       FIG. 3  is a screen shot of Atomic Force Microscopy (AFM) patterns of CdS films deposited on nitride by vacuum evaporation, showing height contrast on the left and phase contrast on the right, constructed in accordance with the principles of the present invention; 
       FIG. 4  is a screen shot of AFM patterns of CdS film deposited on nitride by vacuum evaporation after annealing in hydrogen at 200° C. for 20 min, showing height contrast on the left and phase contrast on the right, constructed in accordance with the principles of the present invention; 
       FIG. 5  is a screen shot of AFM patterns AFM pattern of CdS film deposited on nitride by vacuum evaporation after annealing in hydrogen at 240° C. for 5 min, showing height contrast on the left and phase contrast on the right, constructed in accordance with the principles of the present invention; and 
       FIG. 6  is a screen shot of AFM the microstructure of CdS films deposited on nitride by vacuum evaporation after annealing in NF 3  at 200° C. for 5 min, showing height contrast on the left and phase contrast on the right, constructed in accordance with the principles of the present invention. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   The principles and operation of a method and a system according to the present invention may be better understood with reference to the drawings and the accompanying description, it being understood that these drawings are given for illustrative purposes only and are not meant to be limiting. 
   In one preferred embodiment of the present inventions, CdZnS films have been grown in a 2 inch horizontal reactor of Thomas Swan low pressure MOCVD system. Note that in general, the chemical designation for the film is Cd 1-x Zn x S, where (0&lt;x&lt;1). Then, when x=0, the designation is CdS. 
   Three kinds of substrates were used for the Metal Organic Vapor Phase Epitaxy (MOVPE) growth. There were glass slides; silicon wafers with 100 nm PECVD nitride; and silicon wafers with 100 nm Plasma Enhanced Chemical Vapor Deposition (PECVD) oxide. Metalorganic precursors used for the growth were dimethylcadmium (DMCd), diethylzinc (DEZn) and t-butylmercaptan. Hydrogen purified by diffusion through palladium was used as a gas carrier. The growth processes were performed at total pressures of 100 Torr and 300 Torr and total flow of 2 liters/min. High total flow velocity was used to increase compositional homogeneity of the gas flow. The epilayers were grown at 300° C. at a growth rate of about 1 μm/h. 
     FIG. 1  is a schematic illustration of the electrical contacts  110  on the CdZnS film surface  120 , constructed in accordance with the principles of the present invention. The electrical resistance was measured using two contacts  110  evaporated on CdZnS surface  120 . Surface  120  is shown in a side view  130  and an orthogonal top view  140 . The configuration of the contacts allowed the measurement of the square sheet, resistance. Ti, Al or Ti/Au were used as the contact materials. 
   According to semiconductor theory, resistance depends on temperature as
 
 R ( T ) =R   0 .exp(− E   a   /KT ).  (1)
 
   Temperature coefficient of resistance TCR is defined as:
 
 TCR ( T )=1 /R*dR/dT=−E   a   /KT   2   (2)
 
   Maximum possible activation energy (E a ) is equal to E gap /2. 
   E a  for single crystalline material is equal to E gap /2, where E gap  is the band gap energy. For polycrystalline materials, such as Cd 1-x Zn x S, the additional delta E a  related to grain boundary barrier height is present. Actual activation energy for the material includes the energy of the donor level plus the energy of the grain boundary barrier. 
   The resistance and TCR data of MOCVD deposited films are summarized in Table 1 herein below. The temperature dependence of the resistance showed typical semiconductor behavior (referring now to  FIG. 2 , below). From the temperature dependence of R, the activation energy and TCR were calculated according to formula (1) above. 
   The sheet resistance varies non-linearly with the film thickness, evidently due to quantum effects acting in the nanostructured films. For the 2000 A thick CdZnS films, the R values are about 200-300 kOhm, i.e. suitable for the bolometric measurements. The resistivity was 4-6 Ohm-cm. For the CdS films with the thickness equal or less than 2000 A the TCR was in the range 1.4%-2.4%. 
   That TCR values varied insignificantly for different substrates can be related to the differences in grain size and morphology. The grain size of the 100 nm thick films was about 30 nm for glass substrate and 50 nm for SiO 2 /silicon and Si 3 N 4 /silicon substrates. The roughness of the films was low and fulfilled the device fabrication requirements. RMS of CdS films is found to be 5.5 nm on glass, 4.3 nm on Si 3 N 4 /silicon and 4.4 nm on SiO 2 /silicon. 
   In an alternative preferred embodiment, in order to further raise the TCR, a Zn component was added to the film. According to theory, the addition of Zn should increase the TCR because the E gap  of ZnS is greater than that of CdS. Indeed, even small concentrations of Zn increased TCR by up to 3.7%. However, at the same time, the resistance of the film also increases with Zn concentration. So the Zn concentration should be optimized for each specific application. 
     FIG. 2  is a graphical representation of electrical resistance versus 1/ T for appropriate TCR avg  of CdS on nitride  210  and glass  220 , in accordance with the principles of the present invention. 
   Table 1 presents TCR data for MOCVD deposited CdZnS films: 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
                 
             
             
                 
                 
                 
               Resistance 
                 
             
             
                 
               Material 
               Thickness, A 
               kOhm 
               TCR % 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
                 
               CdS/glass 
               4000 
               20 
               1.0 
             
             
                 
               CdS/glass 
               2000 
               220 
               2.2 
             
             
                 
               CdS/nitride 
               2000 
               330 
               1.8 
             
             
                 
               CdS/glass 
               1500 
               2540 
               2.8 
             
             
                 
               CdS/nitride 
               1500 
               2300 
               2.5 
             
             
                 
               CdS/SiO 2   
               1500 
               2400 
               2.6 
             
             
                 
               Cd 0.94 Zn 0.06 S/glass 
               2000 
               5200 
               3.7 
             
             
                 
               Cd 0.86 Zn 0.14 S/glass 
               1800 
               &gt;20000 
             
             
                 
                 
             
           
        
       
     
   
   In another preferred embodiment, CdS films were deposited by thermal evaporation in vacuum 10-6 Torr. CdS granules were used for evaporation from the crucible, heated resistively by 27 A current. The substrates were at room temperature. The deposition rate was 0.08 μm/min. The thickness was monitored by the deposition time. The substrates were Si wafers covered by nitride film. The TCR values are close to 3%. 
   Table 2 presents the TCR of CdS films deposited on nitride/silicon by thermal evaporation: 
   
     
       
             
             
             
           
             
             
             
           
         
             
                 
             
             
               Resistance 
               TCR 
                 
             
             
               [KOhm/square] 
               [%] 
               CdS thickness, Å 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                370 
               2.38 
               3000 
             
             
                760 
               2.97 
               1500 
             
             
               1400 
               2.83 
               1350 
             
             
               1000-2000 
               2.2-3.7 
               1000 
             
             
               2200 
               3.6  
               800 
             
             
                 
             
           
        
       
     
   
   In a fourth preferred embodiment, the CdS (1350 Å) films were deposited by thermal evaporation as in the previous example. After deposition, the samples were annealed at 200° C. in a hydrogen atmosphere at 500 Torr for 20 min. The sheet resistance dropped from 1.4 MOhm to 400 kOhm, while the TCR changed from 2.8 to 1.6%. Similar results were obtained with annealing at 240° C. in a hydrogen atmosphere at 500 Torr for 5 min. The resistance drop can be explained by better organization of grain boundary regions in the film during annealing leading to the decrease of potential barriers at the grain boundaries, as well as by grain size growth (with reference to  FIGS. 3 ,  4  below). 
   Table 3 shows the values of thickness, thermoelectric coefficient of resistance (TCR) before and after annealing and resistance before and after annealing for a variety of environments, including a vacuum. 
   
     
       
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               presents CdS annealing in different environments: 
             
           
        
         
             
                 
                 
               TCR 
               TCR 
               Resistance 
                 
             
             
                 
                 
               before 
               after 
               before 
               Resistance 
             
             
                 
               CdS 
               annealing 
               annealing 
               annealing 
               after annealing 
             
             
               Annealing Parameters 
               thickness Å 
               [%] 
               [%] 
               [KOhm/square] 
               [KOhm/square] 
             
             
                 
             
           
        
         
             
               Hydrogen/200° C./20 min 
               1350 
               2.2 
               1.6 
               1400 
               400 
             
             
               Nitrogen//200° C./5 min 
               1000 
               2.8 
               1.5 
               2000 
               320 
             
             
               Vacuum//200° C./5 min 
               1000 
               2.8 
               1.3 
               2000 
               180 
             
             
               NF3/200° C./5 min 
               800 
               3.6 
               1.1 
               2200 
               300 
             
             
                 
             
           
        
       
     
   
   In yet another preferred embodiment, the CdS (800 Å) films were deposited by thermal evaporation. After the deposition the samples were annealed at 200° C. in NF3/N2 (1:1) atmosphere at 60 Torr for 5 min. The sheet resistance dropped from 2.2 MOhm to 300 kOhm while the TCR changed from 3.6% to 1.1%. The AFM results show the significant recrystallization and grain growth ( FIG. 5 ). 
   In a sixth preferred embodiment, the CdS (1000 A) films were deposited by thermal evaporation. After the deposition the samples were annealed at 200° C. in a nitrogen environment at 60 Torr for 5 min. The sheet resistance dropped from 2.0 MOhm to 180 kOhm, while the TCR changed from 2.8% to 1.5%. 
   In a seventh preferred embodiment, the CdS (1000 A) films were deposited by thermal evaporation. After the deposition, the samples were annealed at 200° C. in a vacuum environment for 5 min. The sheet resistance dropped from 2.0 MOhm to 180 kOhm, while the TCR changed from 2.8% to 1.3%. It is critical that the high TCR layer does not change in time. In an eighth preferred embodiment the CdS film (1300 Å) was covered by a ZnS film (300 Å). The ZnS layer plays the role of a passivation protective layer for CdS as well as for CdZnS. The TCR was 2.8% and the resistance was 1 MOhm. The data were stable over an observation period of 6 months. 
   In contrast to vanadium dioxide, the Cd 1-x Zn x S films do not exhibit solar bleeching. Solar cells containing Cd 1-x Zn x S, are established devices for use in extreme conditions, such as in cosmic space. 
     FIG. 3  is a screen shot of Atomic Force Microscopy (AFM): patterns of CdS films deposited on nitride by vacuum evaporation, showing height contrast on the left  310  and phase contrast on the right  320 , constructed in accordance with the principles of the present invention. 
     FIG. 4  is a screen shot of AFM patterns of CdS films deposited on nitride by vacuum evaporation after aging in hydrogen 200° C. for 20 min, showing height contrast on the left  410  and phase contrast on the right  420 , constructed in accordance with the principles of the present invention. 
     FIG. 5  is screen shot of AFM patterns of CdS films deposited on nitride by vacuum evaporation after aging in hydrogen 240° C. for 5 min, showing height contrast on the left  510  and phase contrast on the right  520 , constructed in accordance with the principles of the present invention. 
     FIG. 6  is screen shot of AFM patterns of CdS films deposited on nitride by vacuum evaporation after annealing in NF 3  at 200° C. for 5 min, showing height contrast on the left  610  and phase contrast on the right  620 , constructed in accordance with the principles of the present invention. 
   It is to be understood that the phraseology and terminology employed herein are for the purpose of description, and should not be regarded as limiting. 
   It is important, therefore, that the scope of the invention is not construed as being limited by the illustrative embodiments set forth herein. Other variations are possible within the scope of the present invention as defined in the appended claims and their equivalents.