[[delayer:wet]]
 
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delayer:wet [2013/10/20 14:59] – external edit 127.0.0.1delayer:wet [2023/07/28 06:47] (current) – mcmaster
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 Above left: metal floating off during HF etc.  The SiO2 under it was completely eaten.  Right: polysilicon gates that floated off after the SiO2 was undercut. Above left: metal floating off during HF etc.  The SiO2 under it was completely eaten.  Right: polysilicon gates that floated off after the SiO2 was undercut.
  
-An isotrophic etch removes materially equally in all directions.  Most wet etches are anisotropic because they remove any material they touch at constant rate.  However, monocrystaline Si has planes that can be exploited to achieve some anisotropic etching.  If you truly need anisotropic etching you should instead use [[RIE]].  That said, reasonably good results can often be had by etching to just where the metal stops and then etching away the metal with a separate solution.  However, wet etching tends to be less consistent than RIE which still limits its use.+An isotrophic etch removes materially equally in all directions.  Most wet etches are isotropic because they remove any material they touch at constant rate.  However, monocrystaline Si has planes that can be exploited to achieve some anisotropic etching.  If you truly need anisotropic etching you should instead use [[RIE]].  That said, reasonably good results can often be had by etching to just where the metal stops and then etching away the metal with a separate solution.  However, wet etching tends to be less consistent than RIE which still limits its use.
  
 The isotrophic nature of wet etches generally limits wet etching to large feature sizes [JM experience, "Tools and Techniques" 149].  One thing to watch out for is beginning to undercut lower metal layers early from etching through vias.  This can cause entire metal segments to lift off.  Newer processes have this issue less because they use tungsten vias which tend not to etch as easily. ["Tools and Techniques" 148-149] The isotrophic nature of wet etches generally limits wet etching to large feature sizes [JM experience, "Tools and Techniques" 149].  One thing to watch out for is beginning to undercut lower metal layers early from etching through vias.  This can cause entire metal segments to lift off.  Newer processes have this issue less because they use tungsten vias which tend not to etch as easily. ["Tools and Techniques" 148-149]
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   * Notes   * Notes
     * Adding HCl may reduce insoluble oxide bi-products     * Adding HCl may reduce insoluble oxide bi-products
 +
 +==== Pitting ====
 +
 +A common issue seems to be pitting if left in too long. As BOE itself can't etch silicon, it must be an additive. Some evidence suggests this is caused by not cleaning upper (ie metal) layers away. Possibly due to atmospheric oxygen. This also causes issues for staining and similar processes.
 +
 +Recommendation: use phosphoric acid etc to remove upper layers completely before exposing silicon. Clean die and use fresh BOE solution.
 +
 +Below example is on Generalplus GPLB52A24A
 +
 +{{mcmaster:delayer:wet:gp_overetch:dies.jpg?300}}
 +
 +Above: left 40 min etch, right 40 hour etch 130F forced air w/ BOE
 +
 +{{mcmaster:delayer:wet:gp_overetch:angle1.jpg?300}}
 +
 +Above: die at angle, left side lower than right. Right side shows deep pits in focus even though its higher than surface visible at left
 +
 +{{mcmaster:delayer:wet:gp_overetch:s2-dlyr1_01.jpg?300}}
 +{{mcmaster:delayer:wet:gp_overetch:s2-dlyr1_02.jpg?300}}
 +
 +Above: deep staining shows doping much higher than substate now as evidenced by significant focus difference
  
  
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 ===== Al ===== ===== Al =====
  
-Beck reccomends 65% phosphoric acid to remove aluminum: "65%, application at 50C. Etch rate 0.2 mm/min. Very uniform attack, also very gentle to oxides and silicon."  Since a layer is rarely more than 2 um thick it should take just a minute to completely remove the layer.  If you are having problems completely removing it there may be a metal barrier, see "Beck barrier Ti/TiN solution" (TODO: add pictures)+==== Phosphoric ==== 
 + 
 +Beck reccomends 65% phosphoric acid to remove aluminum: 
 + 
 +Ingredients: 
 +  * 65% H3PO4 
 + 
 +Procedure: 
 +  - Heat acid to 50C 
 +  - Drop die into acid 
 +  - Cook until done 
 + 
 +Notes: 
 +  * Rate: 0.2 mm/min 
 +    * Since a layer is rarely more than 2 um thick it should take just a minute to completely remove the layer. 
 +  * "Very uniform attack, also very gentle to oxides and silicon." 
 +  * If you are having problems completely removing it there may be a metal barrier, see "Beck barrier Ti/TiN solution" (TODO: add pictures) 
 + 
 + 
 +==== HCl-H2O2 ==== 
 + 
 +Very aggressive.  Use fresh solution, degrades quickly 
 + 
 +Ingredients: 
 +  * 4 mL HCl 
 +  * 1 mL 35 H2O2 
 +  * 1 mL H2O 
 + 
 +[[http://ssel-sched.eecs.umich.edu/wiki/Public.HCl-based%20Aluminum%20Etchant%20Mixture.ashx|Source]].  They omitted concentrations.  I put the ones I assumed they meant / what I've been using. 
 + 
 +==== Nitric ==== 
 + 
 +[[https://books.google.com/books?id=KXwgAZJBWb0C&pg=RA1-PT495&lpg=RA1-PT495&dq=nitric+acid+aluminum+maximum+corrosion&source=bl&ots=RgHSwpzs2o&sig=Y_gnaMjFzp90bGEBafZxPvtOETQ&hl=en&sa=X&ei=bZe0VJmRCYzdoATtkYFY&ved=0CEMQ6AEwBQ#v=onepage&q=nitric%20acid%20aluminum%20maximum%20corrosion&f=false|Handbook of Corrosion Data, 542]] says "At room temperature, the rate at which nitric acid attacks alloy 1100 exhibits a maximum at a concentration of 20%".  Take this as a baseline for  
 + 
 +Ingredients 
 +  * Approx 20% HNO3 
 +    *  1 mL 70% HNO3 : 3 mL H2O 
 + 
 +Procedure: 
 +  - Heat acid to 80C 
 +  - Drop die into acid 
 +  - Cook until done 
 + 
 +Notes: 
 +  * How long?
  
  
 ===== Au ===== ===== Au =====
  
-Use aqua regia+==== Aqua regia ==== 
 + 
 +Classic recipe.  Will attack other stuff 
 + 
 + 
 +==== Mercury ==== 
 + 
 +Works, but prefer solder due to health reasons 
 + 
 + 
 +==== Solder ==== 
 + 
 +Use flux
  
  
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   * 2 mL NH4OH   * 2 mL NH4OH
   * ~3 min @ 50-55C   * ~3 min @ 50-55C
 +
 +Try to adjust for out of strong h2o2...
 +  * 45 mL 3% H2O2
 +  * 1 mL NH4OH
 +  * unknown time
  
 Notes: Notes:
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   * Seems to also react at room temp.  You should be able to see more vigorous bubbling if its actually reacting.  Its harder to see this at higher temps because the H2O2 will also decompose faster on its own   * Seems to also react at room temp.  You should be able to see more vigorous bubbling if its actually reacting.  Its harder to see this at higher temps because the H2O2 will also decompose faster on its own
   * Does not react with Ti?   * Does not react with Ti?
 +
  
  
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   * 1 mL 40% HF   * 1 mL 40% HF
   * 20-25 @ room temp   * 20-25 @ room temp
 +
 +Alternate:
 +  * 1 mL Whink
 +  * 10 mL 70% HNO3
  
 Notes: Notes:
-  * I haven't used this solution yet+  * 2023-07-23: did a good job near top metal when the other solution wasn't working 
  
 ====== References ======  ====== References ====== 
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   * Wikipedia: Buffered oxide etch: http://en.wikipedia.org/wiki/Buffered_oxide_etch   * Wikipedia: Buffered oxide etch: http://en.wikipedia.org/wiki/Buffered_oxide_etch
   * "DSP-1 emulation": http://board.zsnes.com/phpBB3/viewtopic.php?f=6&t=5868   * "DSP-1 emulation": http://board.zsnes.com/phpBB3/viewtopic.php?f=6&t=5868
 +  * https://www.ee.washington.edu/research/microtech/cam/PROCESSES/PDF%20FILES/WetEtching.pdf 
  
 
delayer/wet.1382281147.txt.gz · Last modified: 2014/02/11 01:40 (external edit)
 
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