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Plastic logic's transistor technology

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    Plastic logic's transistor technology Plastic logic's transistor technology Document Transcript

    •     WHITE  PAPER     Plastic  Logic  Technology  Overview  Beginnings    Over   the   last   three   decades   organic   electronics,   which   is   electronics   based   on   carbon   rather   than  silicon,  has  been  extensively  researched.  In  1998  Professor  Richard  Friend’s  group  at  the  University  of   Cambridge,   UK   published   a   seminal   work   using   an   organic   transistor   to   drive   an   organic   light  emitting   diode1.   Two   years   later   Plastic   Logic   was   founded   to   develop   and   commercialize   the  successes   of   the   work   done   by   Professor   Friend,   Professor   Henning   Sirringhaus   and   their   teams   at  the  Cavendish  Laboratory.    Plastic  Logic  soon  focused  its  activity  on  transistor  arrays  for  displays.    Organic  materials  are  typically  flexible,   lightweight   and   robust.   Plastic   Logic   decided   to   exploit   these   attributes   by   developing   its  arrays   on   a   plastic   base   which   would   then   allow   any   final   display   to   be  lighter   and   more   robust   than  equivalent  silicon-­‐based  products.    At   the   same   time,   teams   of   researchers   began   pushing   for   high-­‐quality   materials   that   would   meet  the  rigorous  demands  of  a  commercial  environment.      Several  leading  materials  companies  started  to   put   serious   effort   into   refining   their   materials   for   use   in   this   new   application   space   and   Plastic  Logic   developed   close   relationships   with   many   industrial   research   teams   to   guide   their   work   and  exploit  the  results  at  the  earliest  opportunity.  By   mid-­‐2004   Plastic   Logic   had   developed   small   area   displays   with   relatively   low   resolution   which  were  extremely  robust,  as  evidenced  by  the  photographs  in  Figure  1  a  .              Figure  1  b:  Demonstrating  the  robustness  of  Plastic  Logic’s  displays  –  note  the  small  bend  radius  Just  over  a  year  later,  in  late  2005,  the  company  had  progressed  its  technology  to  large  area  displays  with  much  higher  resolution  as  shown  in  Figure  2.                                                                                                                                1  Sirringhaus  et  al,  Science  (1998)  Vol  280  page  1741-­‐1744      © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  1  OF  11    
    •     WHITE  PAPER          Figure  2:    An  example  of  Plastic  Logic’s  displays  in  late  2005  By  early  2007,  Plastic  Logic  had  identified  a  site  for  its  manufacturing  facility  in  Dresden,  Germany  and   had   begun   the   factory   build.     Ideally   placed   in   the   heart   of   Silicon   Saxony,   Plastic   Logic   has  drawn   a   high-­‐caliber   team   with   extensive   manufacturing   experience   from   the   surrounding   region,  where  many  silicon  manufacturing  facilities  are  based.  The  teams  in  Dresden  and  Cambridge  worked  closely  together  to  ensure  that  the  transfer  of  the  technology  from  lab  to  fab  would  be  as  smooth  and  as  efficient  as  possible.    Only   eighteen   months   later   the   Dresden   manufacturing   facility   opened   its   doors   and   began  producing  flexible  displays  on  a  scale  never  seen  previously  in  the  organic  electronics  community.     a)   b)      Figure  3:    a)  Aerial  image  of  Plastic  Logic’s  manufacturing  facility  in  Dresden  Germany  and  b)  showing  the  size  of  the  motherplates  used  in  the  factory.  In   parallel   the   company   has   been   ramping   its   product   development,   marketing,   and   business  development   and   activities   in   the   US   to   ultimately   complete   the   transition   of   Plastic   Logic   from   a  small  R&D  company,  spun  out  of  academia,  to  a  product-­‐based  organization  with  the  facilities  and  know-­‐how  to  take  technologies  from  the  lab  bench  to  mass  market.    An  Introduction  to  Plastic  Logic  Technology  Now   that   the   field   of   organic   electronics   is   firmly   on   its   journey   to   industrial   maturity   it   is   important  to   step   back   and   recognize   the   key   components   in   taking   a   small   scale,   academic   activity   and   scaling  it  to  the  realities  of  a  commercial  environment.    A  balance  between  device  performance  and  ease  of  manufacture  must  be  struck  for  commercial  success.    For   the   past   10   years,   Plastic   Logic   has   been   at   the   forefront   of   this   progression—   taking   its   own  organic  transistor  technology  from  a  lab  bench  to  a  high  tech  manufacturing  environment—  and  is      © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  2  OF  11    
    •     WHITE  PAPER  therefore   well   placed   to   discuss   the   considerations   from   both   the   research   and   manufacturing  perspectives.      This   document   will   give   an   overview   of   the   key   considerations   which   frame   the   transistor’s  performance  and  manufacturing  considerations  based  on  Plastic  Logic’s  learnings.  Transistors  Transistors   are   formed   from   three   electrodes,   a   dialectric   and   a   semiconductor.     The   electrodes  control  the  current  flow  by  way  of  the  voltage  applied  to  them.    The  semiconductor  is  the  material  through  which  the  current  flows.  A  schematic  is  shown  in  Figure  4.              Figure  4:  A  generic  top-­‐gate  transistor  in  cross-­‐section.  A  good  transistor  is  analogous  to  a  good  water  tap.     1) When  you  turn  the  tap  on,  water  soon  starts  to  flow  and  as  you  turn  it  on  a  little  more  the   water   flows   faster   until   it   is   soon   flowing   very   fast.     Similarly   for   a   transistor,   the   current,   which  is  a  flow  of  electric  charge,  should  begin  to  flow  once  a  small  voltage  is  applied  and  as   you  increase  the  voltage  the  current  should  increase  until  you  have  a  surfeit  of  current  for   your  application.   2) When   you   turn   the   tap   off,   it   shouldn’t   allow   any   water   to   leak   out.     Similarly   a   transistor   should  not  allow  current  to  flow  when  it  is  off.    In  the  vast  majority  of  display  applications  the  transistors  use  silicon  as  the  semiconductor  because  it  is   a   well-­‐established   technology   that   can   provide   ample   current   to   drive   the   LCD,   OLED,  electrophoretic   or   whichever   other   screen   technology   is   being   used.     However,   silicon   has   its  drawbacks   in   terms   of   cost,   ease   of   device   manufacture   and   fragility.     In   these   areas   organic  electronics   offer   an   advantage.     Made   primarily   from   materials   which   can   be   processed   from  solution,   the   transistors   are   inherently   simpler   and   cheaper   to   manufacture.     Even   though   today  silicon   can   have   higher   performance   than   organic   semiconductors,   there   are   many   applications  where   the   performance   advantage   of   silicon   is   not   required   and   where   an   organic   electronics  solution  is  more  cost  effective.        The  key  metric  of  semiconductor  performance  is  mobility.    This  is  effectively  a  measure  of  the  speed  at   which   the   charge   can   flow   in   the   semiconductor.     The   required   mobility   is   dependent   on   the  application.  The  faster  the  application,  the  higher  the  mobility  needed.  For  a  television,  the  picture  changes   rapidly   and   hence   the   mobility   required   is   high.     Where   the   image   changes   more   slowly,  such  as  in  an  e-­‐reader,  the  mobility  can  be  much  lower.        © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  3  OF  11    
    •     WHITE  PAPER  The  typical  mobility  of  crystalline  silicon  is  on  the  order  of  1000cm2/Vs  but  many    applications  use  poly-­‐crystalline   silicon   (mobility     >50cm2/Vs   )   or   amorphous   silicon   (mobility   ~0.5cm2/Vs)   as   the  performance  is  still  adequate  but  the  cost  of  manufacture  is  greatly  reduced.        Within  organic  transistors  there  is  also  a  mobility  range  available.    Pentacene,  which  is  a  crystalline  material,  can  achieve  mobilities  of  10cm2/Vs  but  it  is  difficult  to  process  on  any  meaningful  scale.    At  the  other  end  of  the  spectrum,  fully  amorphous  polymer  devices  are  simple  to  manufacture.    They  can  be  made  and  driven  in  air,  without  encapsulation,  and  have  a  whole  host  of  attributes  which  are  extremely   desirable   in   a   manufacturing   context,   but   they   can   only   reach   mobilities   of   around  0.05cm2/Vs.     Nevertheless   this   is   still   sufficient   for   a   number   of   applications.   For   example,  electrophoretic   displays,   which   are   used   to   make   e-­‐paper   and   use   reflected   light   rather   than   an  internal   backlight,   can   be   successfully   driven   with   mobilities   in   this   range.   The   mobility   values   of  various  semiconductors  are  summarised  in  Figure  5.     Source:  A.  Salleo      Figure  5:  Mobility  levels  of  various  semiconductors.  Much   is   made   in   the   academic   literature   about   high   mobility   devices   and   often   this   is   the   metric  which  denotes  whether  or  not  a  device  is  a  success.    However,  the  highest  mobility  devices  are  often  made  in  nitrogen  environments  using  toxic  or  expensive  solvents  and  using  processes  which  are  slow  and  inherently  small  scale.  Such  devices  are  of  no  use  in  commercial  products.    Consistent  devices  are   needed,   made   from   materials   which   are   easy   to   manufacture   on   a   large   scale,   at   a   sensible  cost,  with  good  reproducibility  and  which  are  easy  to  process  in  air.    This  is  often  forgotten  in  the  quest  for  headline  mobility  values.      Fortunately,  over  the  last  few  years  there  has  been  increasing  effort  on   parameters   other   than   mobility.   Now   that   materials   manufacturers   are   becoming   more  acclimated   with   industrial   requirements,   materials   are   starting   to   appear   which   are   closer   to  pentacene   in   performance   whilst   retaining   many   of   the   desirable   processing   attributes   of   the  amorphous   materials.     This   development   will   open   up   display   applications   beyond   electrophoretic  into  LCD  and  OLED  displays.    Additionally  this  advancement  will  enable  organic  electronics  use  in  a  number  of  non-­‐display  applications  such  as  logic.            © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  4  OF  11    
    •     WHITE  PAPER  Mobility  isn’t  the  only  factor  which  determines  the  current  that  is  available.  The  size  of  the  transistor  is  also  important.    If  the  transistor  is  large  enough  then  a  high  current  can  be  achieved  even  with  a  low  mobility.    In  practice,  the  space  available  for  the  transistor  is  usually  limited.    For  example,  a  laptop  screen  is  backlit   and   the   light   must   pass   through   the   transistor   array   to   the   user.     The   transistor   is   not  transmissive  and  hence   needs  to  be  as  small  as  possible  if  the  front-­‐of-­‐screen  performance  is  not  to  be   impaired.     In   an   electrophoretic   application,   where   reflected   light   is   used,   the   size   of   the  transistor   will   not   affect   the   user   experience   and   this   substantially   relaxes   the   size   constraint,  allowing  the  devices  to  be  much  larger  and  consequently  allowing  the  transistor  mobility  to  be  much  lower.      There  are  still  limits  however.    For  example,  in  active  matrix  displays  at  least  one  transistor  is  required  to  drive  each  pixel.    Therefore,  in  a  display  with  a  resolution  of  200  pixels  per  inch  all  of  the  requirements  for  the  pixel  need  to  fit  within  a  space  127  µm  x  127  µm  in  size.    In  an  ideal  transistor  the  current  would  begin  to  flow  once  a  small  voltage  has  been  applied  to  the  device   to   turn   it   on.     Usually,   however,   there   is   a   resistance   preventing   current   flow   when   the  voltage  begins  to  be  applied.    This  resistance  is  caused  by  poor  physical  or  electrical  contact  between  the   semiconductor   and   the   electrode,   known   as   contact   resistance,   and/or   by   the   bulk   of   the  semiconductor  hindering  the  charge  as  it  travels  to  the  semiconductor/dielectric  interface  where  the  charge  flow  occurs.      In  order  for  current  to  flow  the  voltage  must  be  increased  to  overcome  the  resistance.  The  size  of  the  resistance  is  especially  important  in  mobile  applications  because  the  greater  the  voltage  that  is  required  to  obtain  a  useful  current,  the  quicker  the  battery  will  run  down.    It  is  therefore  desirable  to  minimize   any   resistance   as   far   as   possible,   by   appropriate   choice   of   materials   and   careful  consideration  of  the  cleaning  methods  and  device  processing  methods  employed.      It   is   also   wasteful   if   a   high   voltage   is   needed   to   turn   the   transistor   off   as   this   also   requires   power  which  will  shorten  the  battery  run  time.    Thus  it  is  preferred  if  the  transistor  is  off  with  no  significant  current   flow   when   no   voltages   are   applied.     Additionally,   a   high   current   flow   with   only   minimal  voltage   increase   is   optimum   so   the   device   should   switch   from   off   to   on   with   only   a   small   applied  voltage.      The   materials   choice   for   each   of   the   components   of   the   transistor   (source,   drain,   gate,  semiconductor  and  dielectric)  can  have  significant  implications  for  its  performance  and  the  relative  ease  that  charge  can  flow.  The  source  and  drain  electrodes  must  be  chosen  so  that  charge  can  flow  easily   from   the   source   through   the   semiconductor   to   the   drain   when   the   transistor   is   on.     The  dielectric   must   also   be   carefully   chosen   as   the   wrong   dielectric   can   reduce   the   device   mobility   by  several   orders   of   magnitude   which   would   render   the   device   worthless.     Plastic   Logic   has   long    realized   the   importance   of   the   dielectric   choice   and   has   extensive   experience   in   matching   the  dielectric   to   the   semiconductor.     Materials   suppliers   are   now   also   seriously   investigating   the  dielectric   selection   to   provide   the   combination   of   dielectric   and   semiconductor  to   device   companies  rather  than  just  providing  the  semiconductor,  which  was  previously  the  case.    From  this  discussion  it  is  clear  that,  when  designing  a  transistor  for  the  mass  market,  mobility  is  only  part   of   the   story.   The   ease   at   which   the   device   can   be   turned   on   and   off   is   also   important   and  depends  on  the  choice  of  materials.  However,  there  are  yet  further  considerations  when  the  leap  is      © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  5  OF  11    
    •     WHITE  PAPER   made  from  the  individual  transistor  to  the  active  matrix  array  for  a  display  application.    For  example,   in  arrays,  device  uniformity  is  key.    It  is  expected  that  devices  will  perform  similarly  to  one  another,   otherwise  visual  differences  may  be  observable  in  the  resultant  display.  Operational  stability  is  also   required   so   that   the   array   continues   to   function   predictably   throughout   its   life,   with   all   of   the   individual  devices  aging  consistently  regardless  of  how  they  have  been  driven.       Active  Matrix  Arrays  for  Display  Applications   Active  matrix  arrays  consist  of  a  series  of  transistors  laid  out  in  a  grid.    The  isolated  gate  line  shown   in  Figure  4  is  extended  to  connect  all  transistors  in  the  same  row  and  the  source  line  in  Figure  4  is   extended   to   connect   all   the   transistors   in   the   same   column.   This   allows   each   transistor   to   be   uniquely   addressed.     These   arrays   can   then   be   used   to   drive   display   media,   for   example,   electrophoretic   media   (such   as   E   Ink),   LCD   or   OLED.   In   the   simplest   architecture,   each   pixel   within   the   display   is   controlled   by   one   transistor   and   if   the   transistor   is   switched   on   then   the   pixel   will   switch   and   otherwise   will   not   switch.   A   schematic   is   shown   in   Figure   6a   with   the   display   pixels   overlaid  in  Figure  6b.  a)   b)   Figure  6:  a)  A  transistor  array  and  b)  Display  pixels  overlaying  the  transistor  array   Voltage   is   applied   to   the   first   gate   line   and   concurrently   each   source   line   in   parallel,   this   is   then   repeated  with  the  second  gate  line  and  so  on  until  all  the  transistors  have  been  addressed  and  all   the   pixels   are   on   or   off   as   required   for   the   image.       Because   the   millions   of   transistors   within   the   array  are  addressed  one  row  at  a  time,  any  one  transistor  is  only  addressed  for  a  very  short  period.     In   the   example   in   Figure   6b,   voltages   are   applied   to   turn   on   the   TFT   at   the   Source-­‐2   Gate-­‐2   intersection  (S2G2)  and  change  the  associated  pixel  to  its  on  state,  which  is  white,  and  then  applied   to  S4G3  and  finally  S2G4  to  change  their  pixel  colors  to  white.  The  remaining  transistors  are  left  in   their  off  state  and  the  pixels  remain  black.     LCD   color   displays   use   this   basic   principle   and   then   use   color   filters   distributed   in   a   pattern   across   the  display  to  give  red,  green  and  blue  pixels  as  well  as  white  ones.    This  methodology  can  also  be   used   for   reflective   technologies   although   there   are   also   other   device   architectures   that   can   be   employed.     The   gate   lines   and   source   lines   running   across   and   down   the   transistor   array   can   form   transistors   other  than  those  in  the  array  if  the  array  is  poorly  constructed.  These  unwanted  transistors,  called   parasitics,   can   cause   the   display   pixels   to   turn   on   when   they   should   be   off.     It   is   important   that       © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  6  OF  11    
    •     WHITE  PAPER  careful  consideration  is  given  to  where  connections  are  routed  and  how  the  devices  are  built  up  so  that  parasitic  devices  are  avoided.      Plastic  Logic  has  extensive  knowledge  in  array  design  to  minimize  the  impact  of  parasitic  devices.    Parasitic  transistors  are  not  the  only  source  of  unwanted  current.    Transistors  within  the  array  can  also  leak  current  to  one  another  so  it  is  important  to  ensure  there  is  no  path  for  the  current  to  travel  between  neighboring  devices.        While   we   have   focused   on   transistors,   these   are   not   the   only   devices   within   the   array   and   the   other  components  must  not  be  neglected.  During  the  time  that  the  transistor  is  not  being  addressed  the  charge  it  produced  during  the  address  time  needs  to  be  retained  until  it  is  next  addressed.    This  is  achieved  by  the  use  of  a  storage  capacitor  which  comprises  two  plates  separated  by  a  dielectric.  The  drain  pad  of  the  transistor  makes  up  one  of  the  plates  of  the  capacitor.  The  cross-­‐section  is  shown  in  Figure  7.    Figure  7:  Cross-­‐section  of  TFT  and  capacitor  combination  The  metric  for  the  capacitor  is  known  as  capacitance.    The  capacitance  is  a  measure  of  the  ability  of  the  capacitor  to  store  charge  and  is  determined  by  the  capacitor’s  area,  the  separation  of  the  plates,    and  a  measure  of  the  dielectric  known  as  the  dielectric  constant.    For   any   given   capacitance   the   area   of   the   capacitor   can   be   reduced   if   the   dielectric   constant   is  increased.    As  space  is  at  a  premium  within  the  array  it  would  be  ideal  to  have  a  dielectric  with  a  high  dielectric   constant   so   that   the   capacitor   can   be   as   small   as   possible.     Unfortunately   most   organic  transistors   have   relatively   small   dielectric   constants,   when   compared   to   inorganic   transistors,   and  consequently  the  capacitor  structure  is  often  larger  than  would  ideally  be  the  case.    The  competing  requirements   of   the   transistor   and   the   capacitor   present   one   of   the   problems   that   has   to   be  addressed   for   success   in   the   displays   market.   There   are   several   routes   to   solve   the   problem,   all   of  which  present  challenges.     1) The   transistor   could   be   shrunk   to   allow   more   space   for   the   capacitor,   although   this   will   increase  the  mobility  requirement.     2) The  dielectric  used  could  have  a  high  dielectric  constant  so  that  the  capacitor  can  be  small,   but  this  will  impair  transistor  performance.            © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  7  OF  11    
    •     WHITE  PAPER   3) A   capacitor   and   the   transistor   could   be   processed   such   that   they   use   different   dielectrics   from  one  another,  although  this  will  certainly  add  complexity  and  consequently  cost  to  the   system.     As  was  previously  stated,  materials  suppliers  have  recently  started  to  realize  that  the  dielectric  must   also   be   carefully   optimized   to   match   the   semiconductor   and   in   so   doing   maximize   the   transistor   mobility.    Materials  manufacturers  also  need  to  extend  this  thinking  and  realize  that  the  transistor  is   not  the  only  component  in  the  array  and  that  developing  a  semiconductor  that  could  work  with  high   dielectric  constant  materials  would  be  very  desirable.       The   array   structure   is   complicated   further   because   the   bottom   capacitor   plate   (the   drain   pad)   needs   to   be   in   direct   contact   with   the   display   media.     As   shown   in   Figure   7,   the   drain   pad   is   underneath   all   of   the   other   layers   so   it   therefore   has   to   be   brought   to   the   top   of   the   stack.   This   is   achieved   by   adding  an  interlayer  dielectric,  making  a  hole  in  the  stack  of  layers  and  adding  a  metal  or  polymeric   conductor  on  top  to  effectively  move  the  bottom  capacitor  plate  from  the  bottom  of  the  stack  to  the   top.     A  generic  repeat  unit  in  the  active  matrix  array  would  therefore  be:  a)   b)     Figure   8:   a)   Plan   view   of   a   generic   repeat   unit   in   an   active   matrix   array,   b)   cross-­‐section   of   repeat   unit.   Thus   once   the   transistors   are   incorporated   into   a   real-­‐world   application   there   are   many   aspects   which  must  be  considered  and  not  just  the  design  and  performance  of  the  transistor  itself.    This  is   true  not  only  in  displays  but  also  in  non-­‐display  applications  such  as  sensors  or  RFID.     Non-­‐Display  Applications   Transistors   can   either   be   p-­‐type   or   n-­‐type   depending   on   whether   they   are   turned   on   by   applying   negative  voltages  or  positive  ones.  For  display  applications  an  active  matrix  array  can  be  produced   using  transistors  which  are  either  all  p-­‐type  or  all  n-­‐type.    Logic  circuits,  however,  are  most  efficient   if  both  n-­‐type  and  p-­‐type  transistors  are  available.     To   date   the   vast   majority   of   organic   transistors   are   p-­‐type   because   p-­‐type   semiconductors   are   the   most  advanced  in  terms  of  our  understanding  and  also  in  terms  of  the  key  performance  metrics  such   as  mobility.    However  n-­‐type  transistors  would  bring  many  advantages  even  into  the  displays  space.           © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  8  OF  11    
    •     WHITE  PAPER  A   display   requires   drivers   in   order   to   address   the   pixels   correctly   and   in   Plastic   Logic’s   case   all   of   the  driving  electronics  which  surround  the  active  matrix  array  are  made  from  silicon.    Some  of  this  could  be  replaced  by  organic  transistors  if  both  p  and  n-­‐type  materials  were  available.    This  would  allow  the  advantages  of  organic  materials,  namely  ease  of  processing,  cost  and  robustness  to  be  utilized  in  more  of  the  system.  Some  companies  are  beginning  to  seriously  develop  n-­‐type  materials  and  Plastic  Logic   is   actively   engaged   in   the   testing   and   development   of   these   materials   to   ensure   they   reach  commercial  viability  as  soon  as  is  practicable.    In   addition   to   n-­‐type   devices,   Plastic   Logic   has   also   given   significant   consideration   into   how   the  devices  are  constructed  so  that  unwanted  capacitances  and  currents  can  be  removed.    Plastic  Logic’s  IP   portfolio   extends   broadly   over   high-­‐resolution   printing   methods,   where   sub-­‐micron   channel  lengths   have   been   demonstrated,   and   fine-­‐feature   patterning   techniques,   both   of   which   help   to  reduce  parasitics  and  improve  the  device  speed.    As  the  transistor  mobility  improves  for  commercially  viable  devices  in  both  p  and  n-­‐type  devices,  and  deposition   methods   enable   fine   features   and   low   parasitics,   it   becomes   possible   for   organic  electronics  to  move  into  other  application  areas  such  as  RFID,  Sensors,  ASIC,  and  smartcards.        Reliability  Of   paramount   importance   when   discussing   any   commercial   application   is   the   reliability   of   the  electronic   components   in   the   product   and   the   reliability   of   the   process   used   to   make   them.     The  product  will  not  be  a  commercial  success  if  the  transistors  stop  working  when  they  are  exposed  to  heat,   light,   water,   or   wear   out   after   being   operated   for   a   few   weeks.     In   the   case   of   displays   this  would  create  ‘dead’  pixels  which  remain  permanently  off  and  in  logic  circuits  it  would  prevent  the  circuit   from   operating   correctly   and   cause   the   product   to   fail.   While   it   would   be   desirable   for   the  transistors  to  always  perform  the  same  way  in  all  environments  and  all  operating  conditions  this  is  unrealistic.    Temperature  and  moisture  will  change  the  device  behaviour  not  only  in  organic  devices  but   in   silicon   and   other   semiconductors   too.     Additionally,   as   with   most   things,   extensive   use   will  cause   degradation   over   time.     When   designing   a   product   it   is   important   to   investigate   the   operation  of   the   devices   in   a   range   of   environments   and   under   a   range   of   operating   conditions   which   are  specific   to   the   application   in   question.     The   changes   to   the   device   performance   caused   by   varying  these   factors   can   then   be   accounted   for   in   the   design   of   the   devices   such   that   it   doesn’t   cause   a  difference   in   the   visual   performance   of   the   display   or   the   operation   of   the   logic   circuit.     When  completing   such   a   design   it   is   important   to   remember   that   it   is   the   performance   of   the   worst  transistor  that  is  of  most  interest.    The  worst  transistor  in  the  display  must  still  be  functional  at  the  end   of   the   product   life   and   therefore   the   worst   transistor   dictates   the   pixel   design.     If   the   product   is  to  reach  its  full  potential  and  thereby  maximize  revenue  for  the  manufacturer,  uniformity  across  all  the  devices  within  the  display  is  key.    The   importance   of   uniformity   is   also   clear   when   considering   how   the   products   will   be   tested   to  ensure   they   are   fit   for   purpose.   It   is   impossible   to   fully   test   every   device   in   every   product   and  therefore  it   is   important   that   the   transistor   behaviour   is   consistent   and   predictable   such   that   a   basic  test  will  show  whether  the  product  will  work  as  expected.      Again  this  highlights  the  importance  of  using  materials  which  can  be  easily  mass-­‐produced  and  are  well  understood  so  that  the  variability  between  devices  is  minimized.          © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  9  OF  11    
    •     WHITE  PAPER  It   is   not   only   variability   in   the   materials   which   can   cause   variation   in   the   device   performance.    Variability  in  the  process  can  have  the  same  effect  and  hence  the  manufacturing  process  needs  to  be  robust  and  repeatable.  An  unreliable  process  will  reduce  yield,  increase  cost  and  make  forecasting  product  availability  difficult.  This  needs  to  be  considered  at  the  outset,  in  the  initial  device  design,  as  a   complicated   and   intricate   process   will   be   harder   to   maintain   than   a   straightforward   and   simple  one.    Manufacturing  The   requirements   and   intricacies   of   manufacturing   are   worthy   of   a   document   in   their   own   right.    Here,  a  couple  of  examples  are  used  to  give  a  flavor  of  some  of  the  considerations  involved  in  the  transition  from  a  lab-­‐based  environment  to  a  manufacturing  one.  When   moving   from   an   R&D   environment   to   manufacturing   every   minutia   has   to   be   validated   and  understood.     Issues   that   affect   a   couple   of   displays   in   the   lab   could   wipe   out   whole   batches   of  displays   in   a   factory,   which   would   be   extremely   costly.     Thus   it   is   important   to   understand   all   the  parameters  so  that  issues  can  be  rectified  quickly  with  minimal  impact  on  production.    As   an   example,   one   major   consideration   is   display   build   time.     In   a   lab,   where   displays   are   being  processed   one   at   a   time,   tight   time   constraints   can   be   accommodated.     For   example,   if   one   layer  cannot  be  exposed  to  air  for  more  than  an  hour  or  one  clean  or  treatment  process  wears  off  after  ten   minutes,   then   displays   can   be   moved   from   one   station   to   another   quickly   in   order   to  accommodate  this  criterion.    In  a  manufacturing  facility  however,  such  tight  time  constraints  cause  complexity   because   displays   are   usually   processed   in   relatively   large   batches   using   automated  equipment,  meaning  that  any  one  display  must  wait  for  all  the  other  displays  ahead  of  it  before  it  goes   through   a   particular   process.     Any   delay   could   potentially   push   large   numbers   of   displays  beyond   the   allowable   time   between   process   steps.     Consequently   any   time   criticalities   need   to   be  fully   understood,   not   only   so   batches   are   processed   through   genuinely   critical   steps   within   the  allotted   time   but   also   so   perfectly   good   batches   are   not   scrapped   for   failing   to   meet   an   arbitrary  time  constraint.    A   second   issue   in   moving   from   the   lab   to   manufacturing   is   how   to   scale   the   processing   of   flexible  substrates  to  a  size  not  previously  used  in  industry.    The  manufacturing  of  organic  electronic  devices  on  flexible  substrates  is  still  in  its  infancy.    Equipment  suppliers  are  used  to  sheet  fed,  glass  based  products   and   their   tools   are   designed   with   rigid,   inflexible   substrates   in   mind.     Plastic   Logic  addressed  this  conundrum  by  laminating  its  flexible  substrate  to  glass  so  that  it  could  be  processed  as   if   it   were   glass.   This   minimized   the   equipment   modifications,   and   removed   the   challenge   from  each   and   every   tool   supplier,   who   might   each   have   different,   and   potentially   mutually   exclusive,  ways  of  addressing  the  issue,  and  moved  it  squarely  back  to  Plastic  Logic.    This  allowed  Plastic  Logic  to  develop  unrivalled  expertise  and  competency  in  the  handling  and  processing  of  flexible  substrates  and  their  lamination  to  glass  and  facilitated  a  deep  understanding  of  how  the  substrate  is  affected  by   factors   such   as   temperature,   chemicals   and   humidity,   which   is   invaluable   information  not   only   at  the  lamination  stages  but  for  all  of  the  other  processing  steps.These  examples  help  to  show  that  to  successfully   progress   out   of   the   lab   and   into   a   factory   there   are   not   only   scientific   considerations,  such   as   how   the   transistor   degrades   or   whether   devices   are   uniform,   but   also   practical  considerations,  which  are  every  bit  as  important  and  which  must  also  be  addressed.        © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  10  OF  11    
    •     WHITE  PAPER  Conclusion  In   a   commercial   environment   it   is   not   enough   to   design   a   transistor   purely   on   the   basis   of   high  mobility.    The  optimum  transistor  is  the  one  which  can  be  processed  simply,  affordably,  consistently,  and  which  has  a  performance  that  is  sufficient  for  the  task  in  hand.    Additionally  the  requirements  of  the   other   components   of   the   system,   for   example   the   capacitor   in   the   display,   must   also   be  accounted   for   right   at   the   outset   of   the   design.   This   ensures   that   the   design   optimizes   the   system  rather  than  any  individual  component.    In  designing  a  system,  Plastic  Logic  understands  the  balance  that   must   be   struck   between   the   myriad   of   influencing   factors,   and   this   is   critical   to   commercial  success.    Plastic   Logic   has   unrivalled   expertise   in   developing   organic   electronics   for   consumer   products   and   in  such   a   rapidly   changing   technology   environment   it   is   vital   to   remain   at   the   forefront   of   research   and  development   for   early   integration   of   new   features   and   hence   is   a   competitive   advantage.     Plastic  Logic   is   devoting   significant   resources   to   the   integration   of   a   compatible   color   technology   and  optimum   front-­‐of-­‐screen   performance.   Plastic   Logic   is   also   focused   on   the   continued   development  of  the  p-­‐type  transistors  in  its  array,  using  materials  with  similar  performance  to  amorphous  silicon.    For  further  cost  benefit  and  feature  enhancement  it  is  also  developing   n-­‐type  transistors  which  will,  when  integrated  successfully,  expand  the  functionality  of  organic  electronics  beyond  the  transistor  array  and  into  the  surrounding  logic  circuits.    In  Plastic  Logic  the  research  teams  are  highly  aligned  with  the  manufacturing  engineers  to  procure  suitable   equipment   that   can   meet   the   challenges   of   mass   manufacture,   both   in   Dresden   and   in  Plastic   Logic’s   planned   second   manufacturing   facility   in   Russia.     Close   alignment   ensures   rapid  inclusion  of  new  advances  into  the  end  product.  This  work  will  ensure  that  Plastic  Logic  continues  to  advance  its  technology  platform  for  the  future.                         Plastic Logic Inc. Headquarters 650 Castro Street, Suite 500 Mountain View, CA 94041 USA Phone: +1 (650) 584-2100 Fax: +1 (650) 584-2101      © 2011  PLASTIC  LOGIC.  ALL  RIGHTS  RESERVED.  THIS  DOCUMENT  IS  PLASTIC  LOGIC  PUBLIC  INFORMATION         PAGE  11  OF  11