Abstract:
A method of preparing a semiconductor film on a substrate is disclosed. The method includes arranging an insulating substrate in a deposition chamber and depositing a semiconductor film onto the insulating substrate using ion beam deposition, wherein a temperature of the insulating substrate during the depositing does not exceed 250° C. The method can produce a thin film transistor. The disclosed ion beam deposition method forms, at lower temperature and with low impurities, a film morphology with desired smoothness and grain size. Deposition of semiconductor films on low melting point substrates, such as plastic flexible substrates, is enables.

Description:
RELATED APPLICATION DATA 
   This application is based on and claims priority under 35 U.S.C. §119 to Korean Application No. 10-2004-0090495, filed Nov. 8, 2004, the entire contents of which are incorporated herein by reference. 
   FIELD OF THE DISCLOSURE 
   The present disclosure relates to methods of forming thin-film transistors. More specifically, the present disclosure relates to methods of depositing metal-oxide semiconductor field-effect transistors on insulating substrates at low temperatures. 
   STATE OF THE ART 
   In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention. 
   Thin-film transistors (TFTs) formed from metal-oxide semiconductor field-effect transistors (MOSFETs) deposited onto one of various substrates enable many electronics applications. For example, semiconductor-based devices on substrates such as quartz and sapphire can form electronic devices such as laser diodes and liquid crystals. 
   Conventional deposition methods to form semiconductor-based devices on substrates include low pressure chemical vapor deposition (LPCVD) and plasma enhanced chemical vapor deposition (PECVD). In conventional LPCVD, deposition temperatures can be greater than 450° C. In conventional PECVD, deposition temperatures can be greater than 350° C. For conventional substrates such as quartz and sapphire, deposition temperatures of 350° C. to 450° C. or higher typically do not adversely impact the substrate or the resulting device. However, some substrate materials can be adversely impacted at the deposition temperatures associated with these conventional deposition methods. 
   Polymeric-based or plastic substrates with deposited high-quality polysilicon films are desirable for new generations of displays (e.g., paper-like display, portable HDTV). However, conventional approaches for preparing polysilicon films include low pressure CVD (LPCVD) and plasma enhanced CVD (PECVD), but these are carried out at temperatures too high for compatibility with plastic substrates (e.g., 450° C. and 350° C., respectively). Thus, sputtering is the conventional approach for depositing silicon films on plastic (presumably magnetron sputtering, where a high intensity plasma is formed directly above and close to the Si target). However, sputtered films contain argon, hydrogen and oxygen impurities. 
   In addition, grain size is a contributing factor to the performance of conventional TFT-type devices. In general, if the grain size is small, the performance of the TFT is close to that of a TFT incorporating amorphous silicon (α-Si), and if the grain size is large enough, the performance of the TFT is close to a single crystalline silicon (Si) device, such as a device on a silicon wafer. Also, in general, grain influences the electron/hole mobility of the TFT, the subthreshold swing, the threshold voltage, and the leakage current of the TFT. As an example, for a glass substrate (&lt;450° C. process temperature), 0.3 micrometer of grain size is commercial technology. 
   Therefore, it is desirable to have a low temperature or room temperature deposition method that can deposit materials that meet or exceed applicable quality standards for the electronics and manufacturing industries. 
   SUMMARY 
   It has surprisingly been found that a desirable film morphology (e.g., desirable smoothness and grain size), similar to the film morphology attainable with LPCVD, but at lower temperature (good for plastic substrate), and with significantly lower impurities than attainable by sputtering can be achieved. 
   An exemplary method of preparing a semiconductor film on a substrate comprises arranging an insulating substrate in a deposition chamber and depositing a semiconductor film onto the insulating substrate using ion beam deposition, wherein a temperature of the insulating substrate during the depositing does not exceed 250° C. 
   Disclosed methods can be used to produce a thin film transistor by, for example, preparing a semiconductor film on an insulating substrate as disclosed herein, forming, in the semiconductor film, a source region, a drain region, and a channel region between the source region and the drain region, and forming a gate electrode adjacent to the channel region 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     The following detailed description of preferred embodiments can be read in connection with the accompanying drawings in which like numerals designate like elements and in which: 
       FIG. 1  shows an exemplary ion beam deposition (IBD) apparatus. 
       FIG. 2  shows, in schematic cross-section, a semiconductor structure including an α-Si film deposited on a substrate. 
       FIGS. 3A and 3B  are scanning electron microscope images showing an amorphous silicon film on glass. 
       FIGS. 4A and 4B  are scanning electron microscope images showing an amorphous silicon film on glass. 
       FIG. 5  is a graph of reflectance as a function of wavelength show UV-Reflectance results for an as-deposited α-Si film. 
       FIG. 6  is a graph of reflectance at 200 nm as a function of laser energy density results for a crystalline silicon film. 
       FIGS. 7A and 7B  show graphs of elemental composition determined by RBS for as-deposited α-Si films deposited by IBD ( FIG. 7A ) and by conventional sputtering techniques ( FIG. 7B ). 
       FIGS. 8A and 8B  show results using secondary ion mass spectrometry (SIMS) on as-deposited α-Si films deposited by IBD and, for comparison, deposited by sputtering. 
       FIG. 9  shows an exemplary semiconductor structure. 
   

   DETAILED DESCRIPTION 
   According to exemplary embodiments, high-purity amorphous silicon (α-Si) can be deposited onto various substrates (e.g., glass and plastic) at low temperature (e.g., room temperature) using ion beam deposition (IBD) of silicon. IBD involves directing a beam of ions from an ion source to a target spaced apart from the ion source. The deposited α-Si films can then be crystallized by laser annealing to form high-quality polycrystalline silicon (polysilicon) films. 
   An exemplary ion beam deposition (IBD) apparatus is shown in  FIG. 1 . The  FIG. 1  exemplary IBD apparatus  100  includes an ion beam source  102 , a target assembly  104  and a substrate assembly  106  housed within a chamber  108 . An optional source  110  for assisting the deposition and/or pre-clean of substrates can also optionally be included. 
   The ion beam source can be any suitable source. In the  FIG. 1  embodiment, ion beam source  102  is a filamentless RF inductively coupled plasma (RFCIP) source. The source  102  includes a gas inlet  120  and a series of focusing grids  122 , such as three focusing grids, across an opening at a projecting end  124 . The source  102  has a diameter of about 20 cm with good beam collimation. 
   The target assembly can be any suitable target. In the  FIG. 1  embodiment, the target assembly  104 , located at a distance A from the source  102 , includes a plurality of targets  130  mounted circumferentially on a rotating mounting device  132 . The targets  130  rotate around axis  134 . The targets  130  are mounted at an angle  2   1  to the source  102  such that material ablated from the targets  130  are directed toward the substrate assembly  106 . An exemplary distance A is about 80 cm. An example of a material for the target  130  is a 99.999% pure polycrystalline semiconductor, such as polycrystalline silicon. Adjusting the angle  2   1  influences the density of the deposited film on the substrate and the purity of the deposited film, e.g., the Ar content. For example, if the angle  2   1  increases from 45 degrees to 90 degrees, the concentration of Ar increases from 0.2 to 0.8%, and the concentration of hydrogen increases from 0.08 to 0.11%. 
   The substrate assembly can be any suitable source. In the  FIG. 1  embodiment, the substrate assembly  106 , located at a distance B from the source target assembly  104 , includes a substrate fixture  140  with a mounting location for a substrate  142 . The substrate fixture  140  both pivots on axis  144  to provide an angle of incidence θ 2  of the material ablated from the targets  130  with respect the normal of the substrate  142  and rotates around axis  146 . An exemplary distance B is about 80 cm. Adjusting the angle θ 2  influences the density of the deposited film on the substrate and the internal stress of the deposited film. It has been observed that the stress level decreases as the angle θ 2  from normal. 
   The optional source  110  for assisting the deposition and/or pre-clean of substrates can be any suitable source. In the  FIG. 1  embodiment, the optional source  110  is a filamentless RF inductively coupled plasma (RFCIP) source and includes a gas inlet  150  and a series of focusing grids  152 , such as three focusing grids, across an opening at a projecting end  154 . 
   A base pressure in the ion beam deposition (IBD) apparatus is about 10 −8  torr. During deposition using the ion beam deposition (IBD) apparatus of  FIG. 1 , a process pressure is about 0.1 mtorr. 
     FIG. 2  shows, in schematic cross-section, a semiconductor structure  200  including an α-Si film  202  deposited on a substrate  204 , such as a silicon wafer or a plastic substrate. An intervening SiO 2  barrier layer  206  is also shown. After deposition, the α-Si film  202  can be crystallized into polycrystalline silicon by any suitable method, such as by laser annealing. 
   Films of deposited α-Si were investigated for morphology, density and composition, including impurity levels. The results are summarized below. 
     FIGS. 3A and 3B  are scanning electron microscope images showing an amorphous silicon film. In  FIG. 3A , the amorphous silicon film has been deposited at room temperature by IBD. In  FIG. 3B , the amorphous silicon film has been deposited at 550° C. by LPCVD. Note that the grain size of silicon is nearly the same under both deposition methods. The observed grain size was up to about 500 nm (±10%), and averaged about 350 nm (±10%). 
     FIGS. 4A and 4B  are scanning electron microscope images showing an amorphous silicon film deposited by IBD on glass ( FIG. 4A ) and deposited by IBD on plastic ( FIG. 4B ). In  FIGS. 4A and 4B , the glass is Corning 1730 and the plastic is polyether sulfer (PES). In both  FIG. 4A  and  FIG. 4B , the amorphous silicon film has been deposited at room temperature by IBD. Note that the grain size of silicon is nearly the same for both substrates and is nearly the same for both deposition by IBD and deposition by LPCVD ( FIG. 3B ). The observed grain size was up to about 500 nm (±10%), and averaged about 350 nm (±10%). 
     FIG. 5  is a graph of reflectance as a function of wavelength show UV-Reflectance results for an as-deposited α-Si film. In this example, the substrate was quartz with the various α-Si/quartz structures formed with an approximate 50 nm thick Si film. In the  FIG. 5  graph, the UV-Reflectance results for a α-Si film deposited by IBD  510 , a α-Si film deposited at 550° C. by LPCVD  520  and a control sample of polycrystalline silicon  530  (for comparison) are shown. The results for the α-Si film deposited by IBD  510  and the α-Si film deposited by LPCVD  520  are virtually indistinguishable across the wavelengths shown, with no crystalline peak observed. In comparison, the results for polycrystalline silicon  530  include two major peaks indicating the polycrystalline structure of that sample. These peaks are related to the band gap for the polycrystalline silicon. In addition, the surface of the α-Si film was very smooth, as indicated by UV reflectance correlated to other samples with known RMS roughness. 
     FIG. 6  is a graph of reflectance at 200 nm as a function of laser energy density results for a crystalline silicon film. To form the sample, as-deposited α-Si film was exposed to from 1 to 100 shots of an excimer laser with energy density varied from 250 to 750 mJ cm −2  to crystallize grains of silicon. In the  FIG. 6  graph, the UV-Reflectance results for a polycrystalline silicon film deposited by IBD and laser annealed with different total annealing energy is shown. The  FIG. 6  traces correspond to 1 shot ( 610 ),  5  shots ( 620 ) and 10 shots ( 630 ). The results indicate that a laser energy density of about 450 to 500 mJ cm −2  produced the largest observed grains and the highest surface roughness while a laser energy density of about 525 to 575 mJ cm −2  produced the smallest observed grains and the lowest surface roughness. Both of these measurements were empirically derived from observations across a laser energy density of 250 to 750 mJ cm −2 , and additional local minima and maxima may be observed across different ranges of laser energy densities. 
   The density of the films were investigated by Rutherford Backscattering (RBS). Table 1 summarizes the results of these measurements on films prepared by the disclosed IBD method and films prepared by conventional sputtering. The results indicate that the IBD prepared films have about a 10% increased density compared to the sputter deposited films. 
   
     
       
             
           
             
             
           
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               Density 
             
           
        
         
             
                 
               Rutherford Back Scattering (cm −2 ) 
             
             
                 
                 
             
           
        
         
             
                 
               Sputter 
               1030E15 
             
             
                 
               IBD 
               1150E15 
             
             
                 
                 
             
           
        
       
     
   
   To investigate the impurity content of films formed by IBD, RBS was used to determine the composition of as-deposited films.  FIGS. 7A and 7B  show graphs of elemental composition determined by RBS for as-deposited α-Si films deposited by IBD ( FIG. 7A ) and by conventional sputtering techniques ( FIG. 7B ).  FIG. 7A  indicates the presence of iron in the film. Iron or another metal associated with the material of components in the IBD apparatus, such as the grid over the source, can be present in the as-deposited α-Si films in amounts greater than 0.005 at. %. Other metals that may be present can include copper or other soft metals. As a comparison, note the absence of such elemental peak in the  FIG. 7B  graph.  FIG. 7A  also indicates a very low presence or an absence of a peak associated with the carrier gas from the deposition process, for example an inert gas such as argon. Test results indicate Ar in an amount of less than 0.2 at. % is present in the IBD as-deposited α-Si films. As a comparison, note the presence of such elemental peak in the  FIG. 7B  graph for sputter deposition, where Ar is present at greater than about 2.4 at. %. 
     FIGS. 8A and 8B  show results using secondary ion mass spectrometry (SIMS) on as-deposited α-Si films deposited by IBD and, for comparison, deposited by sputtering. In  FIG. 8A , the oxygen concentration is show for as-deposited α-Si films deposited by IBD  810  and, for comparison, deposited by sputtering  820 . In  FIG. 8B , the hydrogen concentration is show for as-deposited α-Si films deposited by IBD  850  and, for comparison, deposited by sputtering  860 . The lower oxygen and hydrogen content in the IBD deposited films in comparison to sputter deposited films indicates a higher quality film occurs from the IBD deposition method. 
   Table 2 summarizes results for different impurities in as-deposited α-Si films deposited by IBD and, for comparison, by sputtering. In most cases, at least an order of magnitude improvement is observed for the IBD deposition method as compared to the sputtering method. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Summary of Impurity Content 
             
           
        
         
             
                 
               Element 
               Sputter Method(at. %) 
               IBD Method (at. %) 
             
             
                 
                 
             
           
        
         
             
                 
               Ar 
               2.4 
               0.2 
             
             
                 
               O 
               0.002 
               0.0002 
             
             
                 
               H 
               0.8 
               0.08 
             
             
                 
               Fe 
               0 
               0.05 
             
             
                 
                 
             
           
        
       
     
   
   Process parameter for the two deposition methods are shown in Table 3. 
   
     
       
             
           
             
             
           
             
             
             
           
             
             
             
           
         
             
               TABLE 3 
             
           
           
             
                 
             
             
               Process Parameters 
             
           
        
         
             
                 
               Process Parameter 
             
           
        
         
             
                 
               IBD 
               Sputter 
             
             
                 
                 
             
           
        
         
             
               Base Pressure 
               1 × 10 −8   
               1 × 10 −6   
             
             
               (torr) 
             
             
               Process Pressure 
               0.1 
               5 
             
             
               (mtorr) 
             
             
               Deposition Rate 
               0.7 
               0.5 
             
             
               (Å/sec) 
             
             
               Temperature 
               Room temperature 
               Room temperature 
             
             
                 
               (self heating) 
             
             
               Energy Source 
               AR beam 
               RF power (200 W) 
             
             
                 
               (RF-type 600 V, 300 mA, 
             
             
                 
               tilting angle = 45°) 
             
             
                 
             
           
        
       
     
   
   The disclosed methods can be used to produce a thin film transistor. For example, a semiconductor film is prepared on an insulating substrate arranged in a deposition chamber by depositing a semiconductor film onto the insulating substrate using ion beam deposition. A temperature of the insulating substrate during the depositing does not generally exceed 250° C. A source region, a drain region, and a channel region between the source region and the drain region are then formed in the semiconductor film. A gate electrode is also formed adjacent to the channel region. 
   An exemplary embodiment of a semiconductor structure is presented in  FIG. 9 . Exemplary embodiments of a semiconductor structure  900  comprise a substrate  902 , a gate  904 , a source  906  and a drain  908 . The source  906  and the drain  908  are separated from the gate  904  by an insulator  910 . A channel layer  912  is formed in the device. 
   In exemplary embodiments, the semiconductor film contains less than 0.001% oxygen, less than 1.0% argon, and less than 0.5% hydrogen, in atomic percent. Further, the substrate includes an insulating material, which may be rigid or flexible. A preferred insulating material comprises a plastic material. 
   Although the present invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that additions, deletions, modifications, and substitutions not specifically described may be made without department from the spirit and scope of the invention as defined in the appended claims.