NMOS Fabrication

Boise State University

STEP 1:

Grow Thermal Oxide

Hardmask for Source and Drain Regions

Objective:

Grow a 5000Å layer of SiO2 on a clean, p-type Si wafer. Wafer should be doped with boron and have a resistivity of 1-10Ωcm. This oxide will be used as a hard mask so that the source and drain regions can be doped.

Oxidation Process:

Follow directions posted by furnace or on lab website to perform wet oxidation.

  • Parameters using oxidation calculator for our furnace: 1000C for 1hr 25min flowing 3sccm of O2.

Measure oxide thickness with the Nanospec. Record these values so that oxide etch time can be determined.

STEP 2:

Photolithography Level 1

Pattern Source and Drain Regions

Objective:

Pattern the source and drain regions.

Procedure:

1. Apply HMDS

  • HMDS is an adhesion promoter and helps the photo resist stick to the surface of the silicon
  • Spin at 500 RPM for 5 seconds followed by 5000 RPM for 35 seconds on Spin Coater. This is typically recipe 1 on the spin coater.

2. Apply positive photoresist (SPR-220-1.2)

  • Immediately follow the HMDS with resist
  • Spin at 3500RPM for 30 seconds. This is typically recipe 2 on the spin coater.
  • Soft bake at 90 C for 1 minute to bake out liquid solvents
  • Measure resist with NanoSpec. Should be ~1um thick.

3. Pattern

  • Use ChemFET Mask 1.
  • Follow contact aligner procedures posted by tool or at contact aligner webpage
  • Expose for 2 seconds at 11.8W/cm^2 for a 1um process

4. Develop

  • MF 26A developer for 20-25 seconds
  • Quench in water

STEP 3:

Etch Oxide to Create Hardmask for Source and Drain Regions

Objectives:

We now have patterned resist on our wafers. We need to etch through the oxide that is not covered by resist in order to expose the silicon where the source and drain regions will be located.

Procedure:

1. Hard bake

  • Bake at 150C for 2 minutes  this strengthens resist to help prevent it from being eaten away by the oxide etch

2. Etch oxide

  • Solution: 7:1  NH3F:HF
  • Experimental etch rate: 640A/min

3. Strip resist

  • Acetone soak with agitation, followed by IPA and Methanol rinse

STEP 4:

O2 Descum

Remove any residual photoresist

Objectives:

The source and drain regions have been etched and the resist was removed with acetone. However, there could still be small amounts of resist over the source and drain regions. To remove this, we will perform an O2 descum.

Procedure:

  • Flow 30 sccm of O2
  • Process Pressure 0.5 Torr
  • RF forward power: 150W
  • Time: 1 minute

STEP 5:

Dope the Source and Drain Regions

Objectives:

The source and drain regions have been patterned and are ready to be doped using diffusion.

Theory:

We will be using spin-on dielectric to apply phosphorus to the wafers. Then they will be placed in the diffusion furnace for drive in. This process can be approximated using Fick’s second law.

Procedure:

  1. 10 second BOE dip to remove native oxide from S/D regions
  2. Measure resistivity of wafer prior to doping
  3. Results: 10.6-11 ohm-cm
  4. Spin on dielectric
  5. Static dispense ~1mL
  6. Spin 3000rpm for 10 seconds
  7. Soft bake for 10 min at 180C on oven
  8. Drive-in in diffusion furnace
  9. Flow 40 sccm N2
  10. Flow 10 sccm O2
  11. Temp: 1200C
  12. Time: 1hr

STEP 6:

Strip Oxide

Regrow Field Oxide

Objectives:

The objective of this lab is to strip the oxide layer that was used to as a diffusion mask whey doping the source and drain regions. Then a field oxide (~5000A) needs to be grown.

Oxidation Process:

Follow directions posted by furnace or on lab website to perform wet oxidation.

  • Parameters using oxidation calculator for our furnace: 1000C for 1hr 25min flowing 3sccm of O2.

STEP 7:

Photolithography Level 2

Pattern Gate Regions

Objectives:

With the field oxide grown, the next step is to pattern the gates between the source and drain regions.

Procedure:

1. Apply HMDS

  • HMDS is an adhesion promoter and helps the photo resist stick to the surface of the silicon
  • Spin at 500 RPM for 5 seconds followed by 5000 RPM for 35 seconds on Spin Coater

2. Apply positive photoresist (SPR-220-1.2)

  • Immediately follow the HMDS with resist
  • Spin at 3500RPM for 30 seconds
  • Soft bake at 90 C for 1 minute to bake out liquid solvents
  • Resist was ~1um thick as measured with Nanospec

3. Pattern

  • Put wafer on to contact aligner chuck
  • For 1st level photolithography, align to wafer flat
  • Press “contact” on contact aligner panel
  • Expose for 2 seconds at 11.8W/cm^2

4. Develop

  • MF 26A developer for 20-25 seconds
  • Quench in water

STEP 8:

Etch Patterned Oxide

Hard Bake:

  • 2 minutes at 150C on oven

Strip Oxide/Resist:

  • BOE dip for 7min 45 sec. Etch rate is ~640A/min
  • PR stripper at 80C for 1 min to attempt to remove resist residue
  • O2 descum as detailed in STEP 4

STEP 9:

Grow Gate Oxide

Oxidation Process:

Follow directions posted by furnace or on lab website to perform DRY oxidation.

  • Parameters using oxidation calculator for our furnace: 1025C for 1 hour flowing 3sccm of O2.
  • Should yield ~500A gate oxide.

STEP 10:

Photolithography Level 3

Pattern Contacts

Objectives:

With the field oxide grown, the next step is to pattern the gates between the source and drain regions.

Procedure:

1. Apply HMDS

  • HMDS is an adhesion promoter and helps the photo resist stick to the surface of the silicon
  • Spin at 500 RPM for 5 seconds followed by 5000 RPM for 35 seconds on Spin Coater

2. Apply positive photoresist (SPR-220-1.2)

  • Immediately follow the HMDS with resist
  • Spin at 3500RPM for 30 seconds
  • Soft bake at 90 C for 1 minute to bake out liquid solvents
  • Resist was ~1um thick as measured with Nanospec

3. Pattern

  • Put wafer on to contact aligner chuck
  • For 1st level photolithography, align to wafer flat
  • Press “contact” on contact aligner panel
  • Expose for 2 seconds at 11.8W/cm^2

4. Develop

  • MF 26A developer for 20-25 seconds
  • Quench in water

STEP 11:

Etch Patterned Oxide

Hard Bake:

  • 2 minutes at 150C on oven

Etch Oxide/Resist:

  • BOE dip for 7min 45 sec. Etch rate is ~640A/min
  • PR stripper at 80C for 1 min to attempt to remove resist residue
  • O2 descum as detailed in STEP 4

STEP 12:

Deposit Metal 1

1um of Al was deposited using the Varian Sputter Tool

STEP 13:

Photolithography Level 4

Pattern Metal 1

Procedure:

1. Apply HMDS

  • HMDS is an adhesion promoter and helps the photo resist stick to the surface of the silicon
  • Spin at 500 RPM for 5 seconds followed by 5000 RPM for 35 seconds on Spin Coater

2. Apply positive photoresist (SPR-220-1.2)

  • Immediately follow the HMDS with resist
  • Spin at 3500RPM for 30 seconds
  • Soft bake at 90 C for 1 minute to bake out liquid solvents
  • Resist was ~1um thick as measured with Nanospec

3. Pattern

  • Put wafer on to contact aligner chuck
  • For 1st level photolithography, align to wafer flat
  • Press “contact” on contact aligner panel
  • Expose for 2 seconds at 11.8W/cm^2

4. Develop

  • MF 26A developer for 20-25 seconds
  • Quench in water

STEP 14:

Etch Metal

Hard Bake

  • Bake at 90C for 2 minutes

Metal etch:

  • 1.5 minutes in Al etchant at 50C
  • Al etchant is …

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Boise State UniversityNMOS Process Flow