Abstract:
This invention describes an apparatus, Scanning Localized Evaporation Methodology (SLEM) for the close proximity deposition of thin films with high feature definition, high deposition rates, and significantly improved material economy. An array of fixed thin film heating elements, each capable of being individually energized, is mounted on a transport mechanism inside a vacuum chamber. The evaporable material is deposited on a heating element. The SLEM system loads the surface of heating elements, made of foils, with evaporable material. The loaded thin film heating element is transported to the substrate site for re-evaporation. The re-evaporation onto a substrate, which is maintained at the desired temperature, takes place through a mask. The mask, having patterned openings dictated by the structural requirements of the fabrication, may be heated to prevent clogging of the openings. The translation of the substrate past the evaporation site permits replication of the pattern over its entire surface. A multiplicity of fixed thin film heating element arrays is provided that can operate simultaneously or in sequence. Multi-layered structures of evaporable materials with high in-plane spatial pattern resolution can be deposited using this apparatus. In one version of the invention, the transport of the evaporant-loaded thin film heating elements is accomplished by the use of cylindrical rotors on whose circumference the heating elements are mounted.

Description:
This application is a continuation-in-part of application Ser. No. 10/159,670, filed Jun. 3, 2002 now abandoned.  
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the use of a scanning localized evaporation methodology for processing of multilayer, patterned electronic and photonic devices, such as transistors, sublimable organic light-emitting diodes (OLEDs), photonic band gap structures, and integrated circuits/systems. One example is the fabrication of displays using OLEDs for applications ranging from computer monitors to personal digital assistant (PDAs). 
     2. Description of the Related Art 
     Introduction: 
     A typical display screen comprises of a regular array of color pixels, each of which can be electrically selected to emit red, green and blue colored light, resulting in a desired shade and brightness. Each pixel is comprised of three devices, a red, a green, and a blue light-emitting element. Liquid crystal displays make use of color filters whose transmissions are selectively blocked to produce red, green, blue pixels. By choosing a single device or a combination of the three, a wide range of colors can be produced. Pixels are produced via patterning of one or more thin film layers deposited on a suitable substrate. 
     In the traditional electronics industry, lithographic and etching techniques are used to selectively remove portions of the blanket films, leaving behind the desired pattern.[ 1 ] Recently, display,[ 2 ] electronic[ 3 ,  4 ] and photonic devices,[ 5 ,  6 ] using organic semiconducting materials (of both low and high molecular weight), have shown certain advantages over traditional inorganic materials. These organic materials, due to the chemical sensitivity to both solvents and lithographic procedures, however, require new fabrication methodologies for both deposition and patterning.[ 7 ,  8 ] 
     One of the techniques typically employed is evaporation of these materials through shadow masks.[ 9 ] This process is limited to relatively large feature sizes. For finer features/pixel sizes, smaller pre-deposited patterns of inert resist materials are employed to serve as shadow masks.[ 10 ] In another methodology, films are deposited on substrates on which have been produced three-dimensional pyramidal structures with triangular bases, each face corresponding to the one of the three primary colors.[ 11 ] These techniques, however, have a number of limitations, such as cost associated with background patterning and multi-step batch processing. Until recently, due both to their high purity and to the ease of producing multilayer device structures,[ 12 - 15 ] sublimable organics have been in the forefront of display and transistor development. However, the fabrication of fill color displays through the adaptation of ink-jet printing for polymeric semiconductor has provided an alternate technology.[ 16 - 18 ] This technology requires the use of specialized substrates. These substrates must have indentations, exhibiting controlled wetting characteristics, which serve as micro-containers or wells for localizing the deposited polymeric solution, prior to drying. 
     Forrest et al. [ 19 ] reported a systematic and quantitative study on the design and limitations of OLED-based flat panel displays (FPDs). Among the various addressing schemes used in electronic displays,[ 20 ] direct and matrix addressing are suitable for OLEDs. The direct addressing scheme, where each pixel is connected to an individual driver, can only be used for discrete indicators and simple alphanumeric displays with few characters. In a matrix-addressed display, pixels are organized in rows and columns, and each pixel is electrically connected between one row lead and one column lead. The addressing schemes, where active electronic components are added to the pixels, are called active-matrix addressing;[ 21 ] while those without extra active components are termed passive-matrix addressing.[ 22 ] 
       FIG. 1  shows typical passive (a) and active matrix (b) architectures for full color organic light emitting diode (OLED) flat panel displays (FPDs). The red ( 8 ), green ( 9 ) and blue ( 10 ) electroluminescent (EL) materials  2  shown separately in  FIG. 1   c  and combined as layer  2  in  FIG. 1   a  and  1   b  are typically sandwiched between transparent conducting indium tin oxide (ITO) and metallic cathode electrodes to produce separate red, green and blue light emitting areas, which constitute a full color pixel. The major difference between the passive matrix architecture and the active matrix architecture is in the patterns of the electrodes. For the passive architecture, the cathode  1  and the anode  3  consist of line structures that intersect perpendicularly to define the elements of the full color pixel, any one of which can be activated by powering the row and column defining that element, whether it be red, green or blue. For the active matrix architecture, the emitting materials  2  are sandwiched between the ITO pads  5  and the common cathode  4 .[ 23 ] Not shown in  FIG. 1   b  are the addressable transistors, which connect the individual ITO pads  5  of each pixel element of the full-color display. 
     A typical OLED construction starts with ITO patterns on the substrate  13 , a common Anode Modifying Layer (AML)  12  (ie. copper phtalocyanine), Hole Transport Layer (HTL)  11  (i.e. N,N′-Bis(naphthalen-1-yl)-N,N′-bis(phenyl) red emitting layer  8  (i.e. 4% of 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum(II) (PtOEP) doped within aluminum (III) 8-hydroxyquinoline (Alq 3 )), green emitting layer  9  (i.e. Alq 3  or 0.8% N,N-dimethyl quinacridone doped within Alq 3 ), and blue emitting layer  10  (i.e. lithium tetra-(8-hydroxy-quinolinato) boron (LiBq 4 )), Electron Transport Layer (ETL)  7  (i.e. Alq 3  or bathocuproine (BCP)), Cathode Modifying Layer (CML)  6  (i.e. cesium floride or lithium floride), and Cathode Layers  1  or  4  (i.e. aluminum or magnesium). In reality, some of these layers might need to be different for each art. 
       FIG. 2  describes typical thin film vacuum evaporation method that relies on resistively heated boats or filaments  15  that deposit thin films on substrate  13  through a mask  14 . The deposition source could also be a laser or e-beam heated target  15 . Alternately, sputtering, plasma or glow discharge methods can be employed. All of the above deposition techniques generally require a significant distance to be maintained between source  15  and substrate  13  to obtain the desired film thickness uniformity. 
     SUMMARY OF THE INVENTION 
     This invention describes an apparatus for the close proximity deposition of thin films with high feature definition, higher deposition rates, and significantly improved material economy. The device loads the surface of fixed heating elements, made of thin films (20A-20000A), with evaporable material. The loaded thin film heating element is transported to the substrate site for the re-evaporation. The re-evaporation takes place through a mask onto a substrate maintained at the desired temperature. The mask, which comprises a pattern, dictated by the structural requirements of the device under fabrication, may be heated to prevent clogging. The translation of the substrate past the evaporation site replicates the pattern on its entire surface. The above cycle is herein termed as Scanning Localized Evaporation Methodology (SLEM). A multiplicity of evaporation sites through multiple SLEM fixtures is provided that can operate in parallel. Multilayered structures of evaporable materials with high in-plane spatial resolution can be deposited using this apparatus. 
     In one version of the invention, the transport of the evaporant-loaded heating elements is accomplished by the use of cylindrical rotors on which the heating elements are mounted. The heating element may be electrically powered by using resistive elements. These elements may be made of a pure material, which do not contaminate the evaporants. Refractory thin metal foils graphite and carbon nanotubes composites and doped semiconductor thin film are examples of suitable materials. 
     The features of this invention are:
         (1) An array of reusable, long-lived, thin-film heaters that are capable of evaporating high purity materials,   (2) A means to load and replenish a multiplicity of evaporable materials on the heater array,   (3) A means to monitor the amount of evaporable material deposited on the heater array,   (4) A means to locally heat the evaporable material at the desirable substrate positions through an appropriate mask.   (5) A means of heating the shadow mask to prevent undesirable clogging of the fine features.   (6) The substrate(s) is(are) mounted on a high-speed, high-precision x-y stage with provisions for cooling, providing the ability to deposit compositionally homogeneous thin films uniform in thickness at specific sites.       

     OLED-based full color flat panel displays (FPDs), thin film electronic devices and photonic band gap structures, which conventionally employ multi-batch fabrication processes, can now be realized in one continual in-line vacuum system equipped with multi-segment SLEM rotors. The ability to tightly control both the patterning-resolution and layer thicknesses, particularly in multilevel structures, results in increased throughput while maintaining superior device performance. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows typical passive (a) and active matrix (b) architectures for full color organic light emitting diode (OLED) flat panel displays (FPDs). The red, green and blue electroluminescent (EL) materials are typically sandwiched between ITO (a transparent conductors) and metallic cathode electrodes. (c) Representative “best case scenario” in terms of number of layers for a full color OLED FPD, relying on common AML, HTL, ETL, CML, and cathode layers. In reality, some of these layers might need to be different for each art. 
         FIG. 2  describes a thin film vacuum evaporation method that relies on resistively heated boats or filaments  15  that deposits thin films on substrate  13  through a mask  14 . The deposition source could also be a laser or e-beam heated targets  15 . Alternately, sputtering, plasma or glow discharge methods can be employed. All of the above deposition techniques generally require a significant distance to be maintained between source  15  and substrate  13  to obtain the desired film thickness uniformity. 
         FIG. 3  illustrates the flowchart of the Scanning Localized Evaporation Methodology (SLEM) process. 
         FIG. 4 . shows schematically a typical SLEM apparatus. 
         FIG. 5  shows a three-imensional illustration of a cylindrical SLEM rotor segment, showing loading and re-evaporation through a mask. 
         FIG. 6  shows illustration of a stacked SLEM rotor segment assembly, capable of depositing a number of films to realize a multi-layered thin film structure. 
         FIG. 7  shows an SLEM rotor, configured for co-evaporating two sublimable materials. 
         FIG. 8  shows schematic illustration of a typical deposition cycle to obtain a full color OLED flat panel display described in  FIG. 1 , using a stacked SLEM rotor segment assembly capable of depositing multi-layered structures on a substrate, whose position is controlled by a precision x-y stage. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 3  depicts schematically the Scanning Localized Evaporation Methodology (SLEM) process for depositing thin films onto substrates, which are placed in close proximity to the deposition source. A thin film deposition cycle begins at the loading station, where evaporation of a sublimable material commences on to an array of heater elements. The heater elements are mounted on a first transport mechanism. The thickness of the deposited film on a selected heating element can be determined at the monitoring station, when this heating element is positioned opposite to the monitoring station by the first transport mechanism. The same transport mechanism also brings the loaded heating elements to the deposition site. Here, the thin film is re-evaporated and deposited through a mask onto a substrate. The substrate is mounted on a second transport mechanism that provides the means to move and index it relative to the mask. In the case where the substrate is larger than the mask pattern, the indexed motion of the substrate permits precise replication and registration of the pattern across the full area of the substrate. The un-evaporated material from the heating element surface is finally removed and collected at the retrieval station. The loading, monitoring, deposition and retrieval of evaporable material constitutes a SLEM deposition cycle. Many of such deposition cycles may be used to achieve a desired device structure. 
     In one case the transport mechanism may be a cylindrical rotor as shown in  FIG. 4 . Here, a thin low-thermal mass continuous strip heater, mounted on the circumference of the rotor, is used as an evaporant source for the vacuum deposition of thin films of various electronic materials, notably organic semiconductors. The thin film thus deposited has a thickness ranging between 30 Angstroms to 20,000 Angstroms. The thin film has lower thermal mass than the substrate resulting in fast evaporation due to enhanced temperature for a given electrical input energy. Since the evaporant films are comparable in thickness to heaters, they can be re-deposited on substrates (via a heated mask assembly (in a localized manner and with minimal heat. Conductive materials, such as metals, needed to make electrical contacts to organic semiconductor films, may also be deposited using this method. The rotor is attached to a shaft, which is driven by an external motor. The shaft may be a hollow tube carrying electrical wires and cooling lines to the rotors. The rotor may be of almost any dimension, its radius limited in size only by the dimensions of the vacuum chamber in which it is mounted. The thickness of the rotor disk is determined by such considerations as the size of the mask. Other sizing constraints are imposed by electrical power distribution system and cooling requirements. 
     The rotor  17  supports on its circumference a heating element  19  that can be made of either one continuous strip or of several discrete segments. This heating element  19  is comprised of, but is not limited to, a metallic foil, a carbon nanotube paper, a graphite paper, a doped semiconductor foil, or an electrically conductive fiber composite. Typically, the surface of the heating element  19  is coated with a layer  16  (see also  FIG. 5 ) of a desired material from an evaporative loading source  15 . Layer  16  is re-evaporated at the substrate location from the segment of the heating element  19 , which is powered by appropriately placed electrodes  18 , when the rotor aligns the designated segment with the brushes  20 , contacting the inside of electrodes  18 . The material  16  is deposited onto the substrate  13  through a mask  23  that may be heated to avoid clogging of the openings by the evaporant. The substrate  13  may be cooled to prevent any adverse effects due to an increase in temperature from the nearby heated mask  23  and heating element  19 . 
     The mass of the material  16 , which has been loaded onto the heating element  19 , can be measured by a quartz crystal microbalance thickness monitor placed between the loading and deposition sites. For example, the rate of evaporation for a given electrical power to the heater segment  19  can be periodically measured by evaporating onto the thickness monitor  21  using the set of brushes  22 . Alternatively, a passive technique such as, but not limited to, ellipsometry can be used to continuously measure the thickness of the deposited layer  16  on the heating element  19 . This enables control of both deposition rate and thickness of the evaporated material on the substrate  13 . The material remaining on the heating element  19 , after the deposition of the evaporant on the substrate, may be recovered using the retrieval unit  24 , powered by the set of brushes  25 . 
     The rotor may be constructed from aluminum or its alloys where the conducting electrodes  18  are embedded in the rotor  17 . The electrodes are insulated from the body of the rotor by embedding them in insulating anodized wells in the aluminum rotor or by insulating them using other materials. The high resistivity of the heating element  19 , allows a number of heating zones to be simultaneously energized at the circumference of the rotor, without one heating element interfering with operation of another. The high resistivity of single wall carbon nanotubes (SWNTs) relative to that of tungsten makes it suitable for a localized thermal evaporation heater. In addition, the extremely high thermal conductivity of SWNTs also provides rapid cooling of the SWNT “paper” in regions where current does not flow, thus further localizing the evaporation area Alternatively, one can use electronic multiplexer circuits to power an array of heating elements with fewer electrical connections. 
       FIG. 5  is a three dimensional illustration of the SLEM rotor showing loading a material  16  from a source  15  on to the heater  19 . The re-evaporation is accomplished when the appropriate heater segment is aligned with contacts or brushes  20  and mask assembly  23 . 
       FIG. 6  shows a schematic illustration of an assembly of stacked-rotors capable of depositing a multiplicity of materials, one material per rotor, to realize multi-layered thin-film devices. The rotors  26  are separated by spacers  27 , which can be used for cooling, ancillary drive mechanisms, or as insulating spacers. 
       FIG. 7  shows a schematic illustration of a SLEM rotor capable of co-evaporating two sublimable materials. Two loading sources  28  and  32  are provided to deposit separate materials  29  and  33 , respectively. Here, monitoring devices are shown as  30  for the first material  29 , and  34  for the combination of material  29  and  33 . The respective brushes, delivering power to these monitoring units, are  31  and  22 . The two layers of materials  29  and  33  could be placed side by side on a heater surface or they can be stacked one on top of the other (as shown in  FIG. 7 ), depending on the application. The relative thickness of layers  29  and  33  can be used to provide flexibility in materials composition of the co-evaporated film  35 . Similarly, the composite film  35  is evaporated in a predetermined pattern on the substrate  13  using an appropriate mask  23 . Any excess material  35  remaining on a heating element after re-evaporation is retrieved in a collector  24  by powering the segment contacted by the brushes  25 . 
     The apparent sublimation temperature differences for the two materials  29  and  33  become insignificant at the localized evaporation region  35  due to the small amount of materials present at any time. Co-evaporation may be used to obtain a layer of one material doped with another. 
       FIG. 8  illustrates schematically the typical SLEM deposition cycle to obtain a full color OLED FPD, using a stacked-rotor assembly capable of depositing multi-layered devices on a substrate  13 . Here, the position of substrate  13  ( FIG. 8B ) is indexed with respect to the masks ( FIG. 8C ) and moved by a precision x-y stage. Alternatively, the rotor assembly along with its masks may be translated and indexed above a substrate. Herein, a typical active-matrix addressed OLED display, as shown in  FIGS. 1(   b ) and  1 ( c ) (also shown in  FIG. 8A  for clarity), requires eight different materials ( 12 ,  11 ,  10 ,  9 ,  8 ,  7 ,  6  and  4 ) to be deposited in the indicated sequence, onto selected locations  5 . This is accomplished using an array of eight SLEM rotors (each dedicated to an individual material), like the one shown in  FIG. 6 . The specific arrangement of the eight stationary masks  36 ,  36 ,  37 ,  38 ,  39 ,  36 ,  36 ,  36  located at the rotor site facing the substrate, is referenced to the pixel spacing, as indicated by the location of the ITO pads  5  on the substrate  13 , having in this example 12 rows and 12 columns making a 12×4 RGB array shown in  FIG. 8B . In our scheme, red, green and blue elements, indicated with R, G, B ITO pads  5 , constitute a full color pixel. The configuration of these elements may be positioned linearly along the x-axis ( FIG. 8B) , or in an other manner. 
     Since layers of five materials ( 12 ,  11 ,  7 ,  6 ,  4 ) are deposited uniformly through out the entire substrate, common masks  36  comprised of open windows are employed ( FIG. 8C ). The need to deposit red  8 , green  9  and blue  10  emitting layers at the specified ITO locations requires the use of patterned masks  37 ,  38  and  39  respectively. 
     The substrate  13  is mounted on the x-y stage, which is first scanned following the pattern  40  ( FIG. 8D ). A typical scanning cycle completes the scanning of all the columns of the substrate  13  along the x-axis, in steps equivalent to three ITO pad elements (including their interpad spacing), before advancing a step along the y-axis  40 . This process is repeated until the entire substrate traverses throughout all eight masks. 
       FIG. 8E  illustrates the manner in which the deposition of various evaporants ( 12 ,  11 ,  8 ,  9 ,  10 ,  7 ,  6  and  4 ) progresses as the substrate advances past the rotor discs. The evaporation of various materials can be started and stopped in time significantly shorter than the time required to advance the substrate to the next step. This prevents cross-contamination between pixels and also provides desired thickness uniformity. The required thickness of a particular layer, determined by its function, can be attained by varying the residence time, the rate of evaporation and length of the mask. 
     The novelty of SLEM arises from its close proximity evaporation, the in-situ patterning, and completion of a device structure (consisting of multiple layers) in a single vacuum pump down step. Current projections indicate deposition times is in the range of 3 to 7 minutes for growing a 3″×4″ OLED display, consisting of an array of 270×360 pixels. 
     While the preferred embodiments of the invention have been described, it will be apparent to those skilled in the art that various modifications may be made in the embodiments without departing from the spirit of the present invention. Examples of such modified embodiments, which are within the scope of this invention, include the heater element materials such as nanotube carbon paper with appropriate resistivity (SWNT), and tungsten films. We have described a typical transport mechanism using the example of a cylindrical rotor structure. However, its shape and design can be varied depending on the application. In addition, delivery of power to the heater elements can be realized in a variety of ways, including brushes or multiplexing circuits. Variations are envisioned in the configuration of the evaporation sources, substrates holding fixtures, and mask configurations. Either the heater array or the substrate(s) is mounted on a high-speed, high-precision x-y stage, providing the ability to deposit compositionally homogeneous thin films with high thickness uniformity, in a site-specific manner. 
     Although we have described organic electroluminescent (EL) devices, the SLEM methodology is adaptable to the patterned growth of organic transistors, photonic crystals, and inorganic based technologies required for the fabrication of integrated circuits. 
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