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Center of MicroNanoTechnology CMi

KOH WETBENCH MANUAL

wetbenche PLADE for KOH etching


Attention To be read first:
  • SAFETY OPERATOR MANUAL (HERE) / PROCÉDURE DE SÉCURITÉ POUR OPÉRER SUR LES WETBENCH DU CMI A LIRE OBLIGATOIREMENT (ICI)
  • RESIST IS TOTALLY FORBIDEN INTO THESE BATHES
  • THE KOH ETCH RATE COULD VARY: FOR ACCURATE ETCHING PLEASE CALIBRAT IT BEFORE PROCESSING LIVE WAFERS

Table of contents: CMI

  1. Introduction
  2. System overview
  3. Standard process at CMI
  4. Safety
  5. Modus operandi
  6. Accessory
  7. References

I. Introduction CMI

The KOH is used to etch silicon. Since Si etch rate depends on crystallographic planes it can give an anisotropic etch. For example, <111> planes are etched 200 times slower than <100> planes.

KOH etching profile
Figure 1: Typical profile obtains after Si <100> etching

The silicon etch rate depends on crystallographic orientation (wafers/design), silicon doping and bath characteristics (KOH bath density and temperature).
Etch rate of Si <100> according to the temperature for different density Etch rate of Si <100> according to the KOH  density
Figure 2: Etch rate of Si <100> according to the temperature for different density Figure 3: Etch rate of Si <100> according to the KOH density
At CMI most of the time we work with <100> wafers with a resistivity between 0.1 and 100 Ω.cm but different kind of silicon wafers can be etched.

II. System overview CMI

Wetbench description:

KOH wetbench
Figure 4: Wetbench schematic view

III. Standard process at CMI CMI

As explained previously KOH etching is strongly dependent on KOH density and bath temperature. At CMI these parameters have been selected to optimize etching with a minimum of roughness.

Roughness according to concentration and temperature
Figure 5: Roughness according to concentration and temperature

The 2 next tables give the KOH etching bath performance at CMI. The KOH 40% can be used during night but not the 23% one (auto logout at 22 o'clock).

 

T°C Si Etch Rate (µm/h) Selectivity Si/SiO2 Bath density
Room Temperature - - 1.39
60 21 (±2) 290 1.37
Table 1: 40% KOH bath

T°C Si Etch Rate (µm/h) Selectivity Si/SiO2 Selectivity Si/Si3N4 bath density
Room Temperature - - - 1.21
60 25 458:1 more than 25000:1 1.20
70 42 349:1 more than 25000:1 1.19
80 74 277:1 more than 25000:1 1.18
90 120 204:1 more than 25000:1 1.18
Table 2: 23% KOH bath

NB : The 40% bath density is checked regularly by the CMI staff but you must check the 23% bath level before starting the heating system (cf. §V).

IV. Safety CMI

As for the other wetbench you must respect CMI safety rules. KOH is a base and neutralization contains strong acid (HCl 37%). When you prepare it, you must be very careful to not spill HCl into KOH! Because of the HCl, it is totally forbidden to put metal into the neutralization bath including metallic chucks!

V. Modus operandi CMI

Full dressing is required to operate on the wetbench.
  1. Login on Z5 computer (no login = no heating).
  2. Check the density at room temperature before starting pumping and heating :
    • Use the dedicated labelled densimeter.
    • The density should correspond with values indicated in previous tables.
    • If the density is higher, you may add some water.
    • Densimeter rinsing and drying is required once the control is completed.
  3. Start the recirculation pump.
  4. Start the heating power (that last ~1 hour for the 40% KOH bath to reach 60°C and ~2 hours for the 23% one to reach 90°C).
  5. Once the working temperature is reached :
    • Use again the dedicated labelled densimeter.
    • The density should correspond with values indicated in previous tables.
    • If the density is higher, you may add some water.
    • Densimeter rinsing and drying is required once the control is completed.
  6. You are ready to process your wafers. The etching starts when you see many bubbles coming from Si surfaces (it may take ~1 min to clear native SiO2) . If you really need to use chucks ask to CMI staff for additional information.
  7. If the processing time is long (hours) then label the bath mentioning your name, phone number and expected end time of the process.
  8. When the time is down, rinse your wafers and switch off the bath heating and the bath pumping.
Metals are totally forbidden into the neutralization bath! So if you use metallic chucks don't forget to unmount them after rinsing.
  1. Neutralization to clear potassium residues is mandatory to continue the process in the clean room:
    • Once your wafers / devices are etched and rinse in DI water, put your wafers into HCl 37% bath for 2 hours.
    • Remove your wafers and rinse them.
  2. You may leave your wafers on the wetbench with a labelling to let them drying gently or you may blow with N2 gun.
  3. Logout out from zone 5 computer.

KOH etching example
Figure 6: KOH etching example

VI. Accessory CMI

We have two different chucks to protect one side of the wafers:
Stainless steel chuck Teflon chuck
Figure 7: Stainless steel chuck Figure 8: Teflon chuck
If you really need to use a chuck please ask CMI staff to get instruction.

VII. References CMI