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delayer:wet [2013/09/01 07:34] – mcmasterdelayer: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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     * Adding HCl may reduce insoluble oxide bi-products     * Adding HCl may reduce insoluble oxide bi-products
  
 +==== Pitting ====
  
-====== Staining ======+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.
  
-===== General notes =====+Recommendation: use phosphoric acid etc to remove upper layers completely before exposing silicon. Clean die and use fresh BOE solution.
  
-Most reactions are quoted to work better with a strong cold light.  This evidently favors the decorating reaction over other side reactions.  I haven't done a study to see how much this really matters.+Below example is on Generalplus GPLB52A24A
  
 +{{mcmaster:delayer:wet:gp_overetch:dies.jpg?300}}
  
-===== Dash etch =====+Above: left 40 min etch, right 40 hour etch 130F forced air w/ BOE
  
-Used for both decorating p areas as well as highlighting certain defects.  In general, etch times are very short when used for decorating p areas (say 15 seconds) but it takes much longer to show crystallographic faults (say 12 hours).  Can show epitaxial layers, doped areas and crystal faults [Beck].+{{mcmaster:delayer:wet:gp_overetch:angle1.jpg?300}}
  
-Areas are initially stained brown (say up to 10 seconds).  Longer exposures turns them blue.  Beck says that strongly p-doped areas turn blue after 10-15 seconds [Beck 62] so I imagine that it really means they turn blue faster.+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
  
-{{:delayer:dash:etch_coloring.jpg?300|}}+{{mcmaster:delayer:wet:gp_overetch:s2-dlyr1_01.jpg?300}} 
 +{{mcmaster:delayer:wet:gp_overetch:s2-dlyr1_02.jpg?300}}
  
-Above: samples before and after dash etchant.  Left side: pre-etch, right side: post etch, top: brightfield, bottom: DIC.  The original sample was visible only with DIC and P/N areas couldn't be distinguished.  After decoration its clearly visible with simple brightfield illumination and DIC may now have made it less clear.+Above: deep staining shows doping much higher than substate now as evidenced by significant focus difference
  
-{{:delayer:dash:time_progression.jpg|}} 
  
-Above: left: unstained sample.  Middle: light dash etching.  Right: longer dash etching turns it blue-green.+====== Staining ======
  
 +===== General notes =====
  
-==== For highlighting p regions ====+Most reactions are quoted to work better with a strong cold light.  This evidently favors the decorating reaction over other side reactions.  I haven't done a study to see how much this really matters.
  
-Use this to figure out of an area is N vs P stained. 
  
-"n-doped areas are scarcely attacked. p-doped zones are clearly brought out, and strongly p-doped areas change their colour from brown to blue after only 10-15 seconds...Epitaxial layers are also well shown."  Accelerated by light.  Taking several hours, very slow / one of the slowest etchants.  [Beck]  For an example of staining ROMs see [[http://static.usenix.org/publications/library/proceedings/smartcard99/full_papers/kommerling/kommerling_html|this]]+===== Dash etch =====
  
-The chip should stain within a few seconds, 10-15 gives it plenty of time to make sure that everything sets well.  However, the staining continues to change  color with longer exposure so beware as n regions will also turn.  It should be safe to ultrasonically clean the chip after staining if desired.+{{:delayer:dash:etch_coloring.jpg?300|}}
  
-Equipment: +[[dash|Main page]]
-  * 100 mL PTFE beaker.  Other HF resistant plastics should also work +
-  * 50 mL container to hold DI water +
-  * Clock +
-  * Plastic pipette +
-  * Optional: bright cold light.  Fiber illuminator should work, I use a a high output LED bicycle light +
- +
-Consumables: +
-  * 5 mL or so of Dash etch.  I'm not sure how much is really needed but you should be able to comfterably cover the chip anyway +
-  * De-ionized (DI) water +
- +
-Procedure: +
-  * Clean chip *thoroughly* (ultrasonic acetone-DI suds-IPA).  Instead of dumping the IPA off and drying chip, instead rinse it with DI water.  Do not let the chip ever dry out.  Keep it submerged in DI water for the time being.  If you suspect there is any grime on the chip you must clean it first or it will mask that region and may cause other complications.  Using tap water suds may cause salt contamination, use de-ionized (DI) water +
-  * Rinse out beaker and container with DI water +
-  * Place chip on bottom of plastic beaker (I use 100 mL PTFE).  Keep a few drops of DI water on it to keep it wet.  They should stick with surface tension. +
-  * Fill second container with 50 mL DI water +
-  * Tilt beaker slightly with the chip on the raised side +
-  * Pippette 5 mL Dash etch into lowered side of beaker +
-  * Turn on bright light and point into beaker +
-  * Watch clock to hit the minute mark (or other easily rememberable spot).  Slowly turn beaker 180 such that Dash etch will creep onto the chip without flipping it over +
-  * Let etch for 10-15 seconds +
-  * Dump the DI water into the beaker +
-  * Dispose of most of the solution in the beaker, making sure to not let the chip get dry.  Wash the chip in the beaker a few times +
-  * Vortex chip in beaker for a couple of minutes.  Experience has shown that if not thoroughly washed acid traces can stay on the chip and it will etch/expose over the next few days, sometimes resulting in deep gouges +
-  * Wash in IPA (ultrasound is safe) and blow dry +
- +
-Notes: +
-  *  I'm currently using "Structural etchants" mix since I now have 48% HF.  Other mixtures may work just as well, I haven't revisited them since I got better at cleaning the sample +
-  * Tap water is not de-ionized! +
-  * All solutions/equipment must be copper free: HF solutions will readily deposit copper on the die and create a pesky mask.  Reccomend using a dedicated metal free beaker +
- +
- +
-==== Mask ROM staining procedure ==== +
- +
-This isn't too hard but requires a great deal more attention than a lot of the other procedures on this site: the sample must be handled much more gently and the etching is time sensitive.  I'm guessing that the doping is done a little different way such that it is much shallower than others making it easily damaged +
- +
-Important things: +
-  * The sample must be very clean so as to not mask any etchants +
-  * Do not apply delayering etchant for longer than need be.  It seems to slowly etch Si and/or pull out ions fast enough that it will lead to poor staining if it comes in contact with the implant for too long +
-  * Do not ultrasound the chip while delayering the ROM area.  This causes things to break off more unpredictably and can lead to some areas etching a lot faster than others causing above problem to get worse +
- +
-For normal ROM staining, optimal time to try dash is probably just a little after the polysilicon is lifted off of the ROM area.  Poly may still be attached to the line drivers, don't worry if it is, they aren't important. +
- +
-Experience has shown that this process is somewhat easy to get somewhat working but required some rigor for reliable results.  Not all of these steps may be necessary but each one helps to control specific risks.  I reccomend you do the above "highlight p regions" procedure first to get a basic hang of things. +
- +
-Equipment: +
-  * 100 mL PTFE beaker.  Other HF resistant plastics should also work +
-  * 50 mL container to hold DI water +
-  * Clock +
-  * Plastic pipette +
-  * Optional: bright cold light.  Fiber illuminator should work, I use a a high output LED bicycle light +
- +
-Consumables: +
-  * Buffered oxide etch (BOE) +
-  * 5 mL or so of Dash etch.  I'm not sure how much is really needed but you should be able to comfterably cover the chip anyway +
-    *  I'm currently using “Structural etchants” mix since I now have 48% HF. Other mixtures may work just as well, I haven't revisited them since I got better at cleaning the sample +
-  * De-ionized (DI) water +
- +
-Procedure: +
-  * Decapsulate chip through your method of choice.  Metal damage probably won't effect results too much +
-  * Clean chip *thoroughly* (ultrasonic acetone-DI suds-IPA).  Instead of dumping the IPA off and drying chip, instead rinse it with DI water.  Do not let the chip ever dry out.  Keep it submerged in DI water for the time being.  If you suspect there is any grime on the chip you must clean it first or it will mask that region and may cause other complications +
-  * Rinse out beaker and container with DI water +
-  * Perform etch cycles until the ROM area has no more SiO2 on it.   +
-    * Place chip on bottom of plastic beaker (I use 100 mL PTFE).  Keep a few drops of DI water on it to keep it wet +
-    * Fill second container with 50 mL DI water +
-    * Add 5 mL of BOE +
-    * Vortex chip for 30 seconds (or longer if you are feeling adventuresome).  Do not use ultrasound +
-    * Dump the DI water into the beaker +
-    * Dispose of most of the solution in the beaker, making sure to not let the chip get dry.  Wash the chip in the beaker a few times +
-    * Vortex in IPA and blow dry.  Do not use ultrasound +
-    * Inspect under microscope to see if ROM active area is exposed +
-    * Notes +
-      * Remove metal layers as they are exposed, probably ideally just as you begin to undercut them.  Ideally they probably should be removed before the mask ROM is exposed +
-        * Al: remove using phosphoric acid.  See full Al section below especially for notes on metal barrier which may be more common in implanted chips +
-        * Cu: haven't had to deal with it yet but it could be problematic since it will want to deposite on the Si +
-      * You really should use BOE over HF: BOE doesn't eat metal which prevents hard to remove AlF salts from depositing on the die +
-      * If you are carefully watching it, you should see it change colors (when dry) and then turn clear once again which means its done +
-      * Initially you can probably get away with longer cycles (maybe a minute or two) but make sure to turn it down to 30 seconds max at a time if its already been a few minutes +
-      * Each round takes me about 10 minutes.  I have lost a few chips to trying to save time by etching longer.  Sometimes this saves time but sometimes it destroys the chip and is very frustrating.  You are probably better off biting the bullet and just doing smaller cycles even if it takes you longer +
-      * I experimented with trying to observe the etch under microscope but found it was more hassle than it was worth for various reasons +
-      * There may be a large amount of field oxide left when its done +
-  * Fill second container with 50 mL DI water +
-  * Tilt beaker slightly with the chip on the raised side +
-  * Pippette 5 mL Dash etch into lowered side of beaker +
-  * Turn on bright light and point into beaker +
-  * Watch clock to hit the minute mark (or other easily rememberable spot).  Slowly turn beaker 180 such that Dash etch will creep onto the chip without flipping it over +
-  * Let etch for 10-15 seconds +
-  * Dump the DI water into the beaker +
-  * Dispose of most of the solution in the beaker, making sure to not let the chip get dry.  Wash the chip in the beaker a few times +
-  * Vortex chip in beaker for a couple of minutes +
-    * Experience has shown that if not thoroughly washed acid traces can stay on the chip and it will etch/expose over the next few days, sometimes resulting in deep gouges +
-  * Wash in IPA (ultrasound is safe) and blow dry +
- +
-Notes: +
-  * Because of the field oxide, it can be difficult to get both a good stain on the ROM as well as to stain the general active areas on the same chip.  If you have only one sample, stain the ROM, photograph it, and then use HF to remove the rest of the Fox.  Re-stain the chip and the active areas should stain but you might lose the ROM staining.  You could try masking it if you were really determined +
-  * If you over-delayer the chip it may not show up at first.  You may still be able to get the data to show up, double the stain time and retry, giving up if you accumulate around 4 minutes +
-  * I'm currently using "Structural etchants" mix since I now have 48% HF.  Other mixtures may work just as well, I haven't revisited them since I got better at cleaning the sample +
-  * Tap water is not de-ionized! +
-  * Over-delayering by 1 minute will likely destroy the data.  The lower you can keep that number the crisper the data will be +
-  * All solutions/equipment must be copper free: HF solutions will readily deposit copper on the die and create a pesky mask.  Reccomend using a dedicated metal free beaker +
- +
- +
-==== For crystallographic faults ==== +
- +
-When used for showing crystalgraphic faults, one of the oldest etchants and "not optimal in terms of selectivity and sensitivity", 4-16 hours [Defect Etching in Silicon]  When to use it: when you need good quality and can afford to wait up to a day. +
- +
-Advantages: +
-  * Very good selectivity +
-  * Chromium free [Beck] +
-    * XXX: what issue would Cr cause? +
- +
-Disadvantages: +
-  * Very slow (hours to near day) +
-    * FIXME: how does light intensity play into this? +
- +
- +
-==== "Structural etchants" mixture [Beck 60] ==== +
- +
-Recommended mixture: +
-  * 3 mL 65% HNO3 +
-  * 1 mL 48% HF +
-  * 10-12 mL 98% HAc +
- +
-Rate: +
-  * (111) Si @ 23C: 2 nm / min +
-  * Dislocation substrate etching: up to 24 hours +
-  * Section decoration: 120 seconds with light +
- +
- +
-==== JM Whink mixture ==== +
- +
-Adjusted from above to have roughly same ratio HF to HNO3 +
- +
-Mix: +
-  * 4 mL of stock solution +
-    * 0.5 mL 65% HNO3 +
-    * 27 mL 2.5% HF +
-  * 11 mL Hac +
- +
-Gave poor results in trial run but I suspect this may have actually been from incomplete SiO2 etching. +
- +
- +
-==== JM RustGo mixture ==== +
- +
-Adjusted from Beck to have same solution concentration of HNO3 and HF, then remainder filled with HAc.  The NH4F2 is ignored.  Although I don't know if its optimal, I have gotten good results with this solution. +
- +
-Mix: +
-  * 3 mL 65% HNO3 +
-    * 13% total solution +
-  * 4 mL 12% HF +
-    * 3.2% +
-  * 8 mL HAc +
-    * remainder+
  
 +Good general purpose etching mixture to differentiate p and n doping, reveal implanted mask ROMs, and also has uses in failure analysis.
  
 ===== Staining Etch [Beck 74, 147] ===== ===== Staining Etch [Beck 74, 147] =====
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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
  
  
Line 863: Line 770:
   * 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:
Line 868: Line 780:
   * 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?
 +
  
  
Line 878: Line 791:
   * 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 ====== 
Line 901: Line 819:
   * 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 
  
 
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