Most used substrates

• Si (100), (111) with 15Å native oxide
• Si with 100-200 nm thermal oxide (ThOx)
• MgO (100)
• YSZ (100), (111), (110)
• SrTiO3 (100), (110)
• ITO-coated glass (ITO = indium tin oxide or In2O3:Sn)
• Glasses (Physical Properties and terminology)
  • Fused silica (SiO2)
  • Borosilicate glass (BK7)
  • Soda Lime Glass
  • Corning 1737
  • CaF2
  • FTO (SnO2:F)

See Purchasing supplies for the various suppliers we use.

Acidic Piranha Procedure - A very aggressive cleaning procedure for removing organics from substrates. Note: this is an exception to the “Add Acid” rule. The reaction is highly exothermic and can boil and potentially explode if the peroxide concentration is too high. Substrates should be reasonably clean (i.e. not covered in fingerprints) before going into the solution - large quantities of organic materials will result in violent reactions.

To clean substrate materials like fused silica or Si wafers, it is important to begin with a series of solvent baths. Typically, trichloroethylene (colloquially called trico), acetone, and methanol are used sequentially for 5 minutes each as a sonication bath for one's substrates. This procedure is recommended by Werner Kern, creator of the RCA clean. One may wish to follow these solvent baths with isopropanol; this cleaning method is then called the TAMI clean. It is important to note that for the highest quality surfaces, normal ACS grade solvents are inadequate. HLPC grade or better solvents should be used; these come in a glass bottle typically, and should not be stored in plastic squeeze bottles.

Depending on the particular needs of one's substrate, these solvent baths may be insufficient; be sure that any subsequent cleaning steps are compatible with the substrate being cleaned. For example, don't clean fused silica in HF.

Also, be keenly aware of the chemistry that happens when mixing chemicals before you mix them. For example, a 2 different so called piranha etches are made during an RCA clean. If you prepare either piranha solution and store it in an unvented bottle, it will explode. If you add acids to solvents, an exothermic reactions occurs, sometimes violently. If the solvents are flammable, one would find his or herself covered in exploding acid that is on fire. Don't let this happen to you. Remember, “Do as you oughtta, add acid to watah.” Always add acid to water slowly.

Looking into:

• June-ing Gil (Samsung) 1998
• Robert E. Kennision (International Business Machines Corp) 1975
• BYU n.d.
• Ankur Verma (Indian Institute of Technology) n.d.
• Eric Sundholm (ECE student of Wager) 2009
• Sony 2004
• Werner Kern (Lam Research Corp) 1990
• J. J. Cras (Georgetown U) 1999
• Tadahiro Ohmi (Patent) 2011

Not deemed worth further investigation:

• Esokolov (University of Maryland) 1999
• Paul A. Kittle (Foam Guy) 2004

more …

This section details some wisdom about cleaning of Si surfaces for the purpose of remove oxides to study reconstructed surfaces or to passivate them.

25 July 2011, From Pam Martin, AR's student in response to how to clean Si for STM studies:

It depends on the orientation and what reconstruction you want. I have worked with the following two:

1. For Si(100) 2×1:
   • heat it at 600C for several hours (pressure increases and then drops, we heat til the pressure is again near the base pressure - this can be done overnight if you trust your power supply to be stable)
   • flash to 1200C for 30 sec, then lower to below 900C (around 875C or so, you dont want to go over 900C for a long time or you can build a lot of SiC if there is C present). You want to go quickly to the flash temperature and then go quickly to below 900C after a flash. You may need to monitor the pressure and end the flash earlier if it rises too much. In this case, you can hold it at 875C for a while until the pressure is lower and then do the flashes. We flash the sample on a dipstick in Joe's system and we try to keep the dipstick cooled during sample preparation to ensure that contaminants leave the sample and go to the dip stick and not vice versa.
   • flash again to 1200C for 15 sec once the pressure has recovered
   • if the pressure doesnt increase significantly during the second flash, this may be enough, but you can wait again at 875C and then do another flash for 10-15 sec
   • after the last flash, hold at a little under 900C for at least 5 min
   • slowly reduce temperature (around 1-2 degrees per second) til below 600C
2. For Si(111)7×7:
   • when I was in Berlin, we simply held the sample at 600C, as with the (100) sample, until pressure dropped, then flashed the sample to 1200C a few times and then shut the power completely off after the last flash. This was not done in the same way in Vania's STM because she found that it significantly degraded the lifetime of the holders, due to thermal shock. Instead, we would do 3-4 flashes in a set for 15-30 sec, depending on how long we could do this without the P increasing too much. After the final flash, we would slowly go from 900 to 600C (again, always quickly to 900 and then slowly lower). She would have us hold at 600C for several minutes, 10 min perhaps. I dont know what the purpose of that was. It probably isnt necessary. When we prepared sample's in Joe's system, we did the flashes, slowly reduced from 900 to 600C, then just shut off the power without the hold.
   • For either of these, the flashes can be performed again if the surface doesn't reconstruct well when you image it in the STM. If there is a cold trap that can be filled with LN2 during the sample preparation, this can be helpful to keep the pressure lower and may help the surface stay cleaner.
3. Chemical etching beforehand?
   • Before dicing up a wafer, we do an RCA clean of the whole wafer. Some people think this isn't necessary, but it is probably a good idea if you can do it. When we mount the sample, we first sonicate it 15 min each in acetone, isopropanol, and deionized water, following with a N2 blow dry. They use solvents from the glass bottles, not the squeeze bottles. In the Lyding group, they have an ozone cleaner that they put the sample holder in once the sample is mounted. This helps to reduce carbon contamination and forms an oxide on the surface that is subsequently flashed off. We usually do this for 20 min.
   • In addition to the methods I've done, I know you can also prepare samples using HF. I believe that (111) works out better than (100), as (100) forms large pits, but I would have to look that up if you want me to confirm it. If you are interested in this, I can look into the HF dipping process or find out if anyone in the Lyding group has done this. It results in a H passivated sample so they would need to heat it up in the chamber to get rid of the H for a clean surface.

7/27/2011. Peter Hugger, SK's student, had this to say about cleaning Si to get LEED patterns: Just a little preliminary success to share with all of you:

Please see the attached 6-dot 1×1 LEED pattern from a cleaned Si <111> surface. The light ring in the image is an optical reflection on the flange window–it is not caused by the electron beam.

The procedure we used to achieve this surface is as follows:

1. Base piranha etch (1:1:5 NH4OH:H2O2:H2O); 10min soak at 75 C
2. Cool water ultrasonic rinse, 5min
3. HF etch (1:20 49% HF:H2O); 1min soak
4. Rinse in cool H2O
5. Store in cool H2O during transfer to vacuum chamber (~10min)
6. Blow dry with He
7. Mount in vacuum chamber, flash (~30s) Si to 1000C

The H2O we use is 18 Mohm-cm deionized nanopure water, non de-gassed.

We achieved a “decent” 3-spot pattern without the 1000C flash, but found this step critical for allowing us to see the full 1×1 pattern. Our working hypothesis is that it is the scant presence of an oxide which has, up until now, prevented us from seeing a LEED pattern. We believe a combination of not boiling the Si in H2O after the HF etch plus flashing the Si to 1000C has made the difference.

When asked about the blow dry procedure, and how to clean the nozzle etc,

It's standard He gas from a large compressed gas cylinder. The nozzle system is nothing more than the metallic nozzle we use for leak checking the UHV system. The only reason I used this was that we don't have a source of nitrogen gas in our lab (like usually is supplied in fume hoods) yet I needed to dry our samples before loading them in the vacuum chamber.

Today we have had increased success finding LEED patterns by removing a purposefully grown “volatile oxide” layer from the silicon wafer by similarly flashing the wafer to high temperature in vacuum..even seeing some secondary diffraction spots! Our research will probably continue in this direction. i.e. - thermally blasting the si substrates as a final cleaning step.