A: Standard cleaning procedure

  1. The chemicals used in this cleaning procedure are extremely hazardous. Always wear goggles (face shield), rubber gloves and apron!
  2. Immerse wafers in piranha solution for 10 minutes. Piranha removes organic contaminants by oxidizing them, and metals by forming soluble complexes. Piranha solution is a 5: 1 mixture of H2SO4 with H2O2. The mixture is self-heating and the H2O2 has to be added very slowly. When cool the solution looses its effectiveness. However, it may be refreshed by adding 1 part of H2O2.
  3. Rinse the wafers off in de-ionized (DI) water for 10 seconds in each of the two rinse beakers successively, and then rinse under the faucet with DI water.
  4. Dip in 10:1 buffered hydrofluoric acid (10:1 BHF) for 5 seconds. This "HF dip" removes the native oxide. Note: Clean, bare silicon is hydrophobic and metallic in appearance.
  5. Rinse in DI water for 10 seconds in the two rinse beakers successively, and then rinse under the faucet with DI water.
  6. Spin dry

Note: Both, piranha solution and HF are extremely hazardous.

If exposed to piranha solution:

Immediately remove all affected clothing.

Rinse yourself at the safety shower/eye wash for 15-20 minutes.

Report to lab staff and/or medical department

If exposed to HF:

Flush exposed area immediately with water for 15-20 minutes.

After flushing, apply HF antidote (Ca-based) to neutralize HF effect.

Report to lab staff and/or medical department.

At medical department, exposed areas should be flushed for additional 30-45 minutes and HF antidote should be re-applied

 

B: Resist coating procedure

  1. Dehydrate wafers on a hot plate for 10 minutes at 120 0C
  2. Place wafer onto the vacuum chuck of the spinner. (Make sure the vacuum pump is turned on and the valve at the vacuum buffer is open).
  3. Cover wafer with hexamethyldisilizane (HMDS) and let react for ~ 1 min.
    HMDS is used as an adhesion promoter.
  4. Spin HMDS off.
  5. Dispense Shipley 1813 photo resist onto the center part of the wafer until about 2/3 of the wafer is covered with resist.
  6. Start a spin cycle. The wafer should spin at 3000 RPM for 30 seconds.
  7. Soft bake at 1000 C for 1 minute on the hot plate. Be careful not to use the hot plate for hard bake, which is at a higher temperature!

 

C: Mask alignment and contact printing procedure

  1. Make sure the UV light source is turned on and warmed up for 5-10 minutes.
  2. Measure the power level of the UV light with the photometer by placing the detector in the center of the mask aligner wafer chuck and sliding it underneath the UV source. The output power level (photometer set to "signal") should be between 10 and 12 mW/cm2.
  3. Set the exposure timer to obtain the exposure dose as required for the resist :
  4. Exposure (mJ/cm2) = power level (mW/cm2) x shutter opening time (sec)
  5. Make sure the vacuum pump is turned on and the valve at the vacuum buffer is open.
  6. The setting at the vacuum control for the aligner should be as follows: MAIN VACUUM off; VACUUM CONTROL fully opened and VACUUM LEAK fully closed.
  7. If necessary, clean the mask with acetone, isopropanol and DI water. Blow dry.
  8. Mount the mask in the holder with the chromium side down and the mask ID letters at the top (facing the UV source).
  9. Open the mask door and place wafer on wafer chuck
  10. Center the wafer by aligning the two flats with the left and the top edge of the wafer vacuum chuck. (Make sure the wafer is not on top of the two metal stops).
  11. If necessary adjust separation between mask and wafer
    1. Elevate substrate by turning thumb wheel to the right until soft contact is made. This point can be approximated by raising the substrate until the mask door just begins to lift off its two outer support posts.
    2. From the point of soft contact lower the substrate 25-50 micron (Each graduation is equal to 1 micron)
  12. If alignment of the wafer to the mask is needed, align the appropriate fiducial marks on the wafer with the corresponding marks on the mask in two different locations (e.g. left and right side of the mask):
    1. Make sure the MAIN VACUUM is off
    2. Using the optical microscope with low or medium magnification, bring the fiducial mark on the wafer and on the mask into focus.
    3. By using the left micrometer, position the left substrate fiducial mark on the same horizontal (left-right) line as the left mask fiducial mark.
    4. With the right micrometer, position the right substrate fiducial mark on the same horizontal (left-right) line as the right mask fiducial mark.
    5. Using the micrometer on the right side of the alignment fixture, position a vertical (front-back) substrate fiducial mark on the same line as the vertical mask fiducial mark.
    6. Repeat steps 3-5 until the required alignment is achieved.
  13. After aligning substrate to mask, turn MAIN VACUUM on. Check that the substrate and mask are still aligned. If there has been a shift during the evacuation of the main chamber it may be eliminated by fine adjustment of the parallelism between substrate and mask (See staff).
  14. Slide fixture under light source.
  15. Expose for the preset amount of time by pushing the START bottom on the timer control.
  16. Turn MAIN VACUUM switch to off.
  17. Release vacuum by pushing the vent knob on the mask holder and open mask door.
  18. Before exposing another wafer check that the mask is still clean (sometimes the resist sticks to the mask).

 

D: Resist development procedure

  1. Pour Shipley 352 developer into the dish marked "developer". The solution should be renewed after approximately 3-5 uses.
  2. Dip the exposed wafer into the developer for typically 30-40 seconds (it takes more time if the developer has been used more) and agitate slowly. The reddish liquid forming near the wafer is the dissolved photo resist.
  3. Rinse the wafer underneath the faucet with DI water.
  4. Spin dry.
  5. Inspect under microscope with yellow filter. Record the minimum resolved line width of the resolution test pattern.
  6. If not completely developed repeat Step 2 for 15 seconds.
    If the resolution is not satisfactory, the problem might be the contact printing step.
    In this case, the resist needs to be removed by rinsing with acetone and the whole process has to be repeated. (See staff).
  7. Hard bake the photo resist at 120 0C for 10 minutes on a hot plate. Hard baking cross-polymerizes the resist molecules, making the photo resist physically hard, more adhesive, and less permeable to chemicals.

 

E: Film thickness and etch rate measurement

  1. Break control wafer into approximately 1" square pieces.
  2. Paint on narrow stripes of resist using a wooden toothpick.
  3. Hard bake for 10 min. at 120 0C.
  4. Using the estimated layer thickness and the approximated etch rate, calculate a total etch time.
  5. Immerse the wafer pieces into the etchant and agitate slowly. The immersion time should be different for each piece, covering a range from half the calculated total etch time up to a 100% overetch.
  6. Strip the resist by rinsing the wafer with Acetone.
  7. Rinse with Methanol.
  8. Rinse with DI water.
  9. Blow dry.
  10. Measure the line height with profilometer.
  11. Plot line height versus etch time. Use this data to determine the actual etch rate and the total thickness of the etched layer. Record data.

 

Process step 2: Active area photolithography

  1. Standard clean wafers (see "cleaning instructions").
  2. Standard resist coating (see "resist coating instructions").
  3. Standard photo masking: Mask #1 (ACTV) (see "mask alignment and contact printing").
    Exposure dose should be 180-200 mJ/cm2.
  4. Standard resist development (see "resist development instructions").
  5. Oxide etching:
    1. 5:1 buffered HF (5:1 BHF) will be used to etch the field oxide. Buffered HF is a mixture of NH4F with HF. The etch rate for thermal SiO2 is ~ 1500 Å/min. depending on temperature.
    2. Use control wafer "C1" to determine the field oxide thickness and the actual etch rate of the 5:1 BHF (see "thickness measurement instructions").
    3. Calculate the total etch time needed according to the oxide thickness to be etched plus a 10 % overetch. The overetch is performed for process latitude (i.e., the oxide thickness and the etch rate both may vary across a wafer and among wafers).
    4. Mount the wafer onto the plastic wafer holder.
    5. Dip wafer in water to wet the surface. Because of the surface tension of BHF, air bubbles can sometimes get trapped on the wafer surface if the surface is completely dry, leading to localized areas where the oxide is not removed.
    6. Dip wafer in 5:1 BHF for the length of time determined in Step 6.3.
    7. Immerse in DI water for 10 seconds and then rinse under the faucet with DI water.
    8. Spin dry.
  6. Inspect etching for completion under microscope.
  7. Strip the resist by rinsing the wafer with Acetone.
  8. Rinse with Methanol.
  9. Rinse with DI water.
  10. Spin dry.
  11. Record the minimum resolved line width of the resolution test pattern.

Process step 3: Gate oxidation

  1. Standard clean wafers (see "cleaning instructions"). Include a control wafer without field oxide ("C2").
  2. Load wafers into quartz wafer boat. To improve the uniformity of the oxide film, place two wafers back to back into one slot and leave one slot gap between successive pairs. Put a dummy wafer at each end.
    To avoid cross contamination the quartz ware is used only for dry oxidation.
    Use only clean tools and do not touch any part of the quartz ware with bare hands!
  3. Grow ~ 800 Å of thermal oxide. This step is performed by the lab staff.
    Growth parameters are: 970 0C; O2 flow: 200 sccm ("dry oxidation"); oxidation time: 60 min.
  4. Inspect wafer for uniformity in oxide thickness (variation in color).
  5. Measure oxide thickness on control wafer "C2" using 5:1 BHF as etchant (see "thickness measurement instructions step").

Process step 4: Poly-Si deposition

  1. Standard clean wafers (see "cleaning instructions"). Include a control wafer with field oxide ("C3" ).
    Important: Do not dip wafers in BHF for more than 5 seconds.
  2. Load wafers into quartz wafer boat. Note: To avoid cross contamination, all quartz ware used in the previous gate oxidation step must be replaced by quartz ware designated to poly silicon deposition.
    To improve the uniformity of the poly-Si film, place two wafers back to back into one slot and leave one slot gap between successive pairs. Put a dummy wafer at each end.
  3. Deposit ~ 4000 Å of polysilicon. This step is performed by the lab staff.
    Deposition parameters are: 650 0C; SiH4 flow: 600 sccm; deposition time: 60 min.
  4. Inspect wafer for uniformity in polysilicon thickness (coloration).
  5. Measure polysilicon thickness on control wafer "C2" using a premixed silicon etchant (HNO3:H2O:HF = 64:33: 3). The etch rate for poly-Si is ~ 6000 Å/min. depending on temperature. Record the polysilicon thickness and the actual etch rate (see "thickness measurement instructions step").

 

Process step 5: Gate photolithography

  1. Standard resist coating (see "resist coating instructions").
  2. Standard photomasking: Mask #2 (POLY) (see "mask alignment and contact printing").
    Exposure dose should be 180-200 mJ/cm2.
  3. Standard resist development (see "resist development instructions").
  4. Poly-Si etching:
    1. Mount the wafer onto the plastic wafer holder.
    2. Dip wafers in water to wet the surface. Because of the surface tension of BHF, air bubbles can sometimes get trapped on the wafer surface if the surface is completely dry, leading to localized areas where the oxide is not removed.
    3. Dip the wafer into the 10:1 BHF for 20 seconds to remove any native oxide, which is etched more slowly than polysilicon. Rinse the wafer with DI water.
    4. Calculate the etching time for silicon etching according to the poly-Si thickness to be etched plus a 10 % overetch. (see process step 4).
    5. Immerse the wafer in the silicon etchant (rectangular shaped plastic container marked "silicon etchant") for the calculated time. Watch the color changes as the silicon is etched away.
    6. Immerse in DI water for 10 seconds and then rinse under the faucet with DI water.
  5. Etch oxide in active area until clear in 5:1 BHF.
    1. Calculate the etching time needed for oxide etching according to the oxide thickness in the active area as determined in process step 3. Add a 10% overetch.
    2. Dip wafers in water to wet the surface.
    3. Immerse wafer in 5:1 BHF for the calculated time.
    4. Immerse in DI water for 10 seconds and then rinse under the faucet with DI water.
  6. Inspect etching for completion under microscope.
  7. Strip the resist by rinsing the wafer with Acetone.
  8. Rinse with Methanol.
  9. Rinse with DI water.
  10. Spin dry.
  1. Record the minimum resolved line width of the resolution test pattern.

Process step 6: Source-Drain Deposition (N+)

  1. Standard clean wafers without HF dip. (see "cleaning instructions").
  2. Spin on "Filmtronics Phosphorosilica spin-on-glass" (PSG) at 3000 RPM for 20 seconds. Less that half of a pipette-full of SOG is needed. Include a control wafer without field oxide ("C4")
  3. Bake at 200 0C for 15 minutes in bake oven. Place the wafers on glass slides.
  4. The N+ pre-diffusion is done in the oxidation furnace by the lab staff.
    Settings are: 5 minutes at 1050 0C in an atmosphere of 10% O2 and 90% N2.
  5. Remove the phosphorus glass:
    1. Mount the wafer onto a plastic wafer holder.
    2. Dip wafers in 10:1 BHF for amount of time determined from annealed PSG rate (4700 Å/min) (~20 sec.). Note that wafer may not be hydrophobic due to film left by Spin-On Glass. Piranha cleaning will remove this film.
    3. Immerse in D.I. water for 10 seconds and then rinse under the faucet with DI water.
    4. Spin dry.
  6. Measure resistivity of the control wafer "C4" with the Four-Point Probe.

 

Process step 7: Source-Drain (N+) Drive and Intermediate Oxidation

  1. Standard clean wafers, (see "cleaning instructions"). Including control wafer "C5".
  2. Load wafers into quartz wafer boat. Note: To avoid cross contamination, all quartz ware used in the previous poly-Si deposition step must be replaced by quartz ware designated to the drive-in oxidation. To improve the uniformity of the oxide film, place two wafers back to back into one slot and leave one slot gap between successive pairs. Put a dummy wafer at each end.
  3. The drive-in and wet oxidation is performed by the lab staff:
    1. Turn on the DI water bubbler heater to 99 0C. Check water level.
    2. When the water is boiling in the bubbler (approximately 20 minutes) start wet oxidation program.
    3. Wet oxidation time is 12 minutes at 1050 0C. Wet oxidation (versus dry) is done because it grows oxide faster. Some of the single-crystal silicon of the source and drain and the polysilicon forming the gate are consumed during this oxidation.
    4. The wet oxidation is followed by a 25 minutes anneal at 1050 0C in N2.
  4. On Control wafer "C5":
    1. 5:1 buffered HF (5:1 BHF) will be used to etch the intermediate oxide. Buffered HF is a mixture of NH4F with HF. The etch rate for thermal SiO2 is ~ 1500 Å/min. depending on temperature.
    2. Use control wafer "C5" to determine the intermediate oxide thickness and the actual etch rate of the 5:1 BHF (see "thickness measurement instructions").
    3. Measure resistivity of control wafer "C5" (afetr intermediate oxide is stripped) with Four-Point Probe.

 

Process step 8: Contact Photolithography

  1. Standard resist coating (see "resist coating instructions").
  2. Standard photomasking: Mask #3 (CONT) (see "mask alignment and contact printing").
    Exposure dose should be 180-200 mJ/cm2.
  3. Standard resist development (see "resist development instructions").
  4. Oxide etching:
    1. 5:1 buffered HF (5:1 BHF) will be used to etch the intermediate oxide. Buffered HF is a mixture of NH4F with HF. The etch rate for thermal SiO2 is ~ 1500 Å/min. depending on temperature.
    2. Calculate the total etch time needed according to the oxide thickness (step 7.4) to be etched plus a 10 % overetch.
    3. Mount the wafer onto the plastic wafer holder.
    4. Dip wafer in water to wet the surface. Because of the surface tension of BHF, air bubbles can sometimes get trapped on the wafer surface if the surface is completely dry, leading to localized areas where the oxide is not removed.
    5. Dip wafer in 5:1 BHF for the length of time determined in Step 6.3.
    6. Immerse in DI water for 10 seconds and then rinse under the faucet with DI water.
    7. Spin dry.
  5. Inspect etching for completion under microscope.
  6. Strip the resist by rinsing the wafer with Acetone.
  7. Rinse with Methanol.
  8. Rinse with DI water.
  9. Spin dry.
  10. Record the minimum resolved line width of the resolution test pattern.

 

Process step 10: Metal Photolithography

  1. Standard resist coating (see "resist coating instructions").
  2. Standard photomasking: Mask #4 (METL) (see "mask alignment and contact printing").
  3. Exposure dose should be 180-200 mJ/cm2.
  4. Standard resist development (see "resist development instructions").
  5. Metal etching:
    1. Pour 800 ml of Aluminum etchant into glass beaker. Heat to 50 0C, using a water bath placed onto a hot plate . Al-etchant has a composition of (approximately) 80% phosphoric acid, 10% H2O, 5% acetic acid, and 5% nitric acid.
    2. Mount the wafer onto the plastic wafer holder.
    3. Immerse wafer in water to wet.
    4. Immerse the wafer in aluminum etchant until the aluminum is removed completely (the endpoint is determined optically, see staff for help). Be sure to keep your wafer moving to replace chemical at the wafer's surface.
    5. Bubbles will form on the surface of the wafer as the etchant removes the metal. Bubbling will subside as etch reaches completion. Etch rate of Al is ~100 Å/sec at 50 0C; it is slower at lower temperature.
    6. Immerse in DI water for 10 seconds and then rinse under the faucet with DI water.
    7. Spin dry.
    8. Inspect etching for completion under microscope.
  6. Strip the resist by rinsing the wafer with Acetone.
  7. Rinse with Methanol.
  8. Rinse with DI water.
  9. Spin dry.
  10. Measure line width of resolution test pattern.
  11. Measure the height of the Aluminum over A: field oxide and B: poly-Si.

Process step 11: Sintering

This step is done in furnace tube at 400 0C in forming gas (90% N2, 10% H2) for 20 minutes. Sintering does two things. It allows the aluminum and silicon to interdiffuse, forming a good contact. It also allows hydrogen atoms from the forming gas to diffuse into the silicon and tie up surface states at the oxide channel interface.