Wafer Cleaning Procedures

 

The avoidance of contamination is extremely important to semiconductor processing and the wafers will undergo many cleaning operations in the course of the process.  Most of the processing will be done in the clean-room environment.  The wafers must be kept covered as much as possible and constant attention is required to prevent them from coming into contact with contaminated surfaces.  The success of the process will depend largely on your ability to keep wafers free from contamination.  Remember that impurity levels, which are typically measured in terms of atoms per cubic centimeter, can have very significant effects.

 

1.1    Caro’s Etch (or Piranha)

 

The Caro’s etch is a general cleaning procedure.  It is especially useful for removing hardened photo resist.

 

The wafers are immersed in a hot mixture of hydrogen peroxide (H2O2) and sulfuric acid (H2SO4).  This can range from a 1:1 ratio by volume to a 1:3 ratio by volume.  The standard process is 1:1, but 1:3 is more aggressive.  This is followed by a thorough rinse in deionized (DI) water. Then the wafers are spun dry and are ready for the next processing step.  The following step by step procedure can be used for 5” wafers in the 7.5”x7.5”x8.5” quartz tank.

 

  1. Start logbook entry.
  2. Use Trionic chemical gloves when handling cleaning equipment
    1. Note that sulfuric acid will aggressively attack most organic and polymeric materials including the chemical gloves.  When the gloves are exposed to sulfuric acid, the affected area will turn brown.  Take care to rinse the gloves as often as is practical during the process.
  3. Move small hotplate from top of spin rinse dryer to general process station in fume hood
    1. This hotplate does not heat the solution.  Caro’s etch is exothermic, so it is self heating.  The hotplate only serves to protect the plastic surface from being burned by the hot bath.
  4. Prepare the solution:
    1. Rinse quartz tank with DI water before use.
    2. Place quartz tank on hotplate.
    3. Add 2500 ml H2SO4
    4. Then add 2500 ml H2O2
    5. H2O2 will remain active in bath for about 30 minutes.
  5. Place wafers in Teflon carrier.
    1. Verify that water is available for step 4 before placing wafers in solution.
    2. Immerse wafers in solution.
    3. Leave immersed for 20 minutes.
  6. Fill QDR with DI water (enable process).
    1. Remove wafers and place in quick dump rinser (QDR).
    2. Run dump rinse process.
    3. Blow-dry one wafer with N2 gun.
    4. Place other wafers back in DI water.
    5. Inspect wafer using microscope.
    6. Repeat etch, if necessary.
  7. Dry wafers.
    1. 8” wafers can be loaded into spin rinse dryer (SRD).
    2. Close door and start cycle.
    3. All other wafers must be dried with N2 gun.
  8. Clean bench.
    1. Aspirate Caro’s Etch solution using acid/base aspirator.
    2. Rinse quartzware tank three times with DI water
    3. Clean surface of hotplate, remove from bench, and replace on top of SRD.
    4. Rinse entire bench well with DI water spray.
    5. Verify that everything is off.
  9. Complete logbook entry.

 

 

1.2    Standard RCA Clean for 5” (8") RCA bench.  Note:  For the 8" RCA Bench, use the quantities in parentheses ()

 

An RCA clean is required immediately before any high temperature process step.  This is necessary to avoid diffusion of surface contaminants into the wafers.  After an RCA clean, wafers must be loaded into a furnace within 24 hours.  If more than 24 hours pass after an RCA clean, the wafers must be considered contaminated and may not be loaded into a furnace.  The RCA clean is one of many effective processes that are currently in use at semiconductor processing facilities.

 

The wafers are first immersed in a hot mixture of ammonium hydroxide (NH4OH), hydrogen peroxide (H202) and deionized (DI) water in a ratio of 1:1:5.  This removes traces of organic residues and when done on exposed silicon surfaces, traps ionic contaminants such as sodium and potassium in a thin oxide.  Wafers are then rinsed in a quick dump rinser (QDR) which provides a fast method of rinsing, allowing wafers to be moved quickly from one step to the next.

 

Next, the thin layer of silicon dioxide (SiO2) is removed by dipping in a dilute hydrofluoric acid (HF).  Another QDR cycle follows. 

 

Wafers are then immersed in a mixture of hot hydrochloric acid (HCL), H2O2, and DI water. The ratio is again 1:1:5.  This mixture is effective for removal of ionic contaminants, especially metals.  Wafers are again rinsed in the QDR.  A final HF dip and QDR step is optional.  Then the wafers are spun dry (or dried with N2 gun) and are ready for the next process step.


Note:  If performing this procedure on the 8" RCA Bench, use the numbers in bold print and in parentheses ()

 

1.             Start logbook entry.

2.             Wear gloves when handling cleaning equipment.

    1. Keep gloves clean – rinse frequently with DI water.
    2. Safety goggles must be worn at all times.
    3. Make sure bottles are tightly capped before storage.

3.             Rinse all tanks and beakers once with DI water.

4.             Add 10 (14) liters of DI water to each RCA1 and RCA2 tank

5.             Add 2 (3) liters of NH4OH to RCA1 tank

6.             Turn on temperature controller for RCA1 and set to heat bath to 75 degrees C.

    1. Note that heaters and sensors must be covered by liquid

-    The feedback loop controls the temperature of the liquid in the tank, not the temperature of the heater.  If the level of liquid in the tank is insufficient to cover the sensors and the heaters, the control loop will be unstable and will likely damage the heaters and/or tanks.  The liquid level sensors should not allow the heaters to turn on if the level is not at a sufficient starting point, and should turn the heaters off if level falls too low during operation.

    1. Wait about 10 minutes for solution to heat to 75 °C.

7.             Add 2 (3) liters of HCl to RCA2 tank

    1. Avoid allowing fumes of HCl and NH4OH to mix.

8.             Turn on temperature controller for RCA2 and set to heat bath to 75 degrees C.

    1. See note in step 6 about temperature control.

9.             Load RCA clean Teflon carrier with wafers.

    1. Do not touch carrier with hands.
    2. Do not set Carrier on bench surface.
    3. Wafers must be relatively clean to start.  This process will not remove photo resist or other heavy residues. Excessively contaminated wafers will contaminate the cleaning equipment. Excessive particulate contamination, including silicon dust from scribing operations, may be redeposited on wafers during cleaning.

10.         Add 2 (3) liters of H2O2 to RCA1 bath once it has reached stable temperature

    1. Note that H2O2 life in this bath is about 40 minutes.  Once H2O2 is depleted, this solution will etch silicon (undesirable).

11.         Place carrier and wafers in NH4OH mixture for 10-15 minutes.

    1. Agitate initially by withdrawing and reimmersing carrier 2 or 3 times.
    2. Leave handle attached to carrier.
    3. Rinse gloves.
    4. Fill, dump, and refill dump rinse tank with DI water while waiting.

12.         Rinse wafers in QDR tank.

13.         Turn off heater for RCA1

14.         Mix 20:1 HF dip if necessary (commonly, the HF in the tank will suffice)

a.      Add 2.5 (10) liters of DI water to HF tank marked for RCA use.

b.      Add 250 (1000) ml of 49% HF.

c.      Add 2.5 (10) liters of DI water to HF tank. (Provides mixing action).

d.      If precise control of oxide removal is required, use more dilute HF. Etch rate is approximately linear with concentration.  At room temperature, the following rates may be expected:

-    20:1 125 angstroms/min

-    50:1   50 angstroms/min

-    100:1 30 angstroms/min

15.         Dip wafers in HF solution for 60 seconds.

16.         Rinse wafers in QDR.

    1. Note that steps 16 and 17 should be done as quickly as possible to avoid growth of native oxide outside of RCA2 solution.

17.         Add 2 (3)  liters of H2O2 to RCA2 tank once temperature has stabilized.

18.         Place carrier and wafers in HCl mixture for 10-15 minutes.

a.      Agitate initially by withdrawing and reimmersing carrier 2 or 3 times.

b.      Leave handle attached – use caution, fumes from bath will condense on handle.

c.      Rinse gloves with DI water.

19.         Rinse wafers in QDR.

20.         Turn off RCA2 heat controller.

21.         Optional step: dip wafers in HF solution for 60 seconds.

    1. Rinse wafers in QDR afterward

22.         Dry wafers in SRD or with N2 gun.

23.         Place dry wafers in clean, dry RCA Clean Box for transport to next process.

24.         Clean bench.

a.      Aspirate solutions out of RCA1 and RCA2 tanks.

b.      Rinse RCA1 and RCA2 tanks with DI water three times.

c.      Rinse entire bench well with DI water spray.

d.      Verify that everything is off and that all chemicals and wafer carriers are stored properly.

25. Complete logbook entry.