Field Oxide Cymos Process

EE 432/532

Field oxide – CyMOS process

Jan. 25, 2016

Group 10

Nicholas Anderson

Andrew Mun

Minh Nguyen

Sivaranjani Devarakonda

Lab instructor – Wei Le

Overview

•In the first lab session safety protocols and use of personal protective equipment i.e. PPE were discussed and demonstrated followed by explanation about the working principles of the different equipment in the NSF Lab and the process flow of the CyMOS fabrication.

•In the second lab, wet oxidation of the wafers was performed. In the third lab session, thickness of the oxide formed was measured using a filametric systems. Then photolithography was performed for P-well formation. Before the oxidation, a standard clean is required for avoiding undesirable chemical impurities on the wafer.

•In this lab report, the operations of Standard clean, wet oxidation followed by photolithography for the P-well formation and subsequent etching process are explained.

•All the fabrication processes were carried out following the SOP and the CyMOSprocess traveler.

1. Starting wafers

• There are 10 wafers in total, including one wafer for each of the four group members (Device wafer), two spare wafers, and four test wafers (TW).
• Each wafer is oriented in the < 100> direction, and is slightly N-type with a dopant as Phosphorus.The resistance of each wafer was calculated with the 4-point probe machine. The specifications are summarized:

Number of wafers: 4 Device wafers, 4 Test wafers, 2 Spare wafers

Resistivity: 1-10 Ω∙cm

Dopant: N-Phosphorus

Diameter: 3 inch

Thickness: 356-406 um

Resistance: 20-25 Ω

1. Standard Clean

The RCA standard clean, as explained briefly above, is used to clean the wafers of contaminants prior to oxidation or dopant diffusion. It is important to clean the wafer before the diffusion processes otherwise the contaminants may diffuse into the silicon at the high temperatures required for diffusion. Solvents can clean oils, inorganic/organic residues but solvents themselves leave their own residues. Therefore, a two-solvent method was used. The basic steps for the process are as follows:

• The wafers were rinsed in a basic solution of ammonium hydroxide (NH4OH+H2O2) to clear the wafers of organic contaminants.
• Afterwards, the wafers were rinsed in DI H20 (cascade rinse) to remove any basic solution left.
• Then the wafers were washed in hydrofluoric (HF) acid solution. HF can remove any native SiO2 from the wafer substance.
• Following which the wafers were rinsed in the acidic solution (HCl) to clear the wafers of any ionic contaminants.
• Finally, the wafers were again rinsed in the cascade rinse to clear any acidic solution

Afterwards the wafers were put in the Spin Rinse/Dry (SRD) machine to remove any liquid from the surface prior to oxidation.

1. Wet Oxidation

Basic chemical equation:

• Before oxidation: After the RCA Clean process, the furnace was set to 800 °C and a Nitrogen flow of 1 Standard Liter Per Minute (SLPM) was established. This is referred to as the furnace’s standby state. Use the plastic tweezers to put the wafer which is dealt with standard clean process between guard wafers and put the boat back into the tube. Use glass rod to push the boat to middle of oven. The boat was slowly pushed in at a rate of 1 inch every 12 seconds to prevent the wafers from breaking due the sudden temperature change compared to the ambient.

• During oxidation: A target oxide thickness of 0.250 µm was chosen, for an oxidation temperature of 1100 °C. After attaining this temperature, 200 sccm of nitrogen was released through the tube, along with steam, for 12 minutes, to grow the appropriate oxide thickness.
• After oxidation: The bubbler was switched off after the oxidation time of 12 minutes was elapsed. Then the ramp down step at 800 °Cwas completed overnight. The wafers were pulled out from the furnace at a rate of 1 inch every 12 seconds under a dry N2 flow of 1slpm.

Then, the thickness of the wafers was measured using the filametric system.

Wafers after oxidation

4.Photolithography

After the initial oxidation process, photolithography using the first mask (P-Well for NMOS device), was completed. The basic steps were as follows:

• Spin HMDS chemical to act as an adhesive (20 seconds at 4000 rpm)
• Spin photoresist (AZ5214E-IR) for 40 seconds
• Prebake at 95°C for 40 seconds to harden the resist
• Expose the wafer for 156 seconds using the stepper
• Develop the exposed region for 55 seconds in the MIF developer
• Cascade rinse for 3 minutes
• Dry and inspect the wafer to ensure devices are correct (serves as a check to validate the alignment)
• Post bake the wafer at 120°C for 2 minutes.
• Etch the SiO2layer to open the window for doping
• Strip photoresist

5.Oxide Etching and Photoresist Stripping

To etch the SiO2, the wafers were put in buffered oxide etchant (BOE)for a total time of 8 minutes and 5 seconds. This was calculated by considering the etching rate and the oxide layer thickness. The wafers were repeatedly checked for hydrophobicity when dipped in DI H20 when about 7 minutes elapsed to ensure that over etching is avoided.

To strip the photoresist, the wafers were washed in “dirty” acetone bath for 3 minutes. This bath removes majority of the photoresist. The wafers were then put in a “clean” acetone bath that removes any remaining resist. The wafers were then rinsed in methanol to remove any acetone remaining on the wafers, and put through cascade rinse and / dried in SRD.

Figure below shows the wafers up through the photolithography and etching stages.

Wafers after photolithography and etching

1. Results

a)Wet Oxidation

The Filmetrics system uses laser to measure the thickness of particular layer in silicon wafer. We used the software to directly measure the thickness. The average thickness of wafers was around 280 nm, the following figure shows the map of thickness for one of the wafers.

Oxide thickness map in nm

1. Appendix

In the experimental process, we put the bare wafer into the 1100°C furnace for12 min 20 sec with wet oxidation process.
Our desired thickness is 0.2500 µm, therefore we want to setup our time for wet oxidation as 12.2709min.

When T = 1100, to = 0 µm,

from the equation above, we got our
B/A = 2.8952µm/hr
B = 0.5289µm^2/hr

Then, we can calculate the time from equation:

if we put tox = 250 µm, and to = 0, and B/A and A into the equation,
we get time 12.2709min

Excel sheet for calculating the time for wet oxidation

The copies of process traveler sheets showing the various values of conditions used are attached.