PI2611 Polyimide Recipe  v 1.3, 3 July 2001

Change Log
1. Remove sanding step- increased Tear stress from 70 MPa to 200 MPa
   (greater than 6 um polyester strength)
2. switched to PI 2611 thin, soft baked layer for mask

*** 50 um wide flexures with 4.5 um thick polyimide ***

see data sheet at www.hdmicrosystems.com

warning- wear coat, latex gloves, goggles and work under fumehood
with polyimide


o pre-heat vacuum oven to 200 degrees C (max temperature ~ 1 hour)

o (if not already in small container, thaw PI2611 and pour into small
   plastic bottle. cap tightly after use) replace PI2611 in freezer
  * warm to room temperature - I suggest a warm water bath for the small
   bottle. Cooler temperature will be viscous and too thick *

_____________________ mask layer + patterning  ____________________

o prepare slide with double sided GelPak X4 to hold SS

o attach 12 um stainless, clean debris from SS with acetone on 
  Q tip until tip is clean

o turn on vacuum for spin coater and position slide. Make sure slide is 
  securely held

o pour PI2611 from small bottle in center of SS, being careful not to
  introduce bubbles, let sit ~ 1 minute to partially spread out
  
o spin on KW4A timer 1: 18 sec, 2:00 on dial
               timer 2: 50 sec, 3:00 on dial  (4.5 um thick polyimide)
  [nonuniform] timer 2: 50 sec, 10:00 on dial  (2.5 um thick polyimide)

o place slide+gelpak+stainless steel in oven for 2 minutes.
  SS will curl up and release from gel pak. Pull SS off of gelpak with
  tweezers, and remove gelpak from oven so it doesn't burn. Polyimide
  will still be tacky, so don't stick anything on it.

o place stainless steel on shelf (sticky side up). Seal door, apply vacuum,
  (to 25" Hg). This should keep toxic fumes out of lab. Heat at
  200 degrees C for 30 minutes, ``Soft bake''.
  Turn on vacuum pump, open inlet partially to exhaust fumes up
  fumehood. (Maybe 2-3 minutes?). Remove SS piece.

o thin polyimide is easily cut with the laser on green, low power ~500 50X,
  or high power ~600 at 20X for ~150 um flexures. Edge of polyimide
  should be clean.  Blast just polyimide, not SS.
  Cuts for flexures must be very clean and smooth to avoid rough spots
  which reduce tear strength.

o repeat procedure above to apply PI 2611 on second side, then soft bake.

_____________________ hard bake  ____________________

   (This step may be necessary before etching to provide strength during
    agitation.)

o Sandwich SS+polyimide piece between 2 glass slides to ensure uniform
  heating. Place on hotplate.  Turn on hotplate heat to #9, 
  ``hard bake'' for 30 minutes. 
  (Do not place cold glass slides on hotplate at high heat- they shatter,
  instead, ramp temperature.) 
  After soft bake, polyimide does not stick to glass. 
  Ramp temperature slowly as absorbed water in polyimide can lead
  to bubbling of film. Perhaps preheat in oven to 200 C?
  This hard bake step seems to increase strength 3-4X compared to softbake.
  (This is hot enough to burn/ignite paper and melt solder so be careful).


_____________________ etching  ____________________

o put S.S. in small beaker with ferric chloride, use magnetic stirrer
  at 3-4, hotplate on #2-3 for 3-10 minutes.
  Check every 2-3 minutes- pull out with plastic tweezers, rinse in 
  flask of tap water. Ferric chloride does not seem to affect polyimide.
  Magnetic stirrer will tear weak flexures. 1 cm ``hard baked''
  flexures survived ok.