EE 432/532Field oxide – CyMOS process

Feb. 1, 2016Group 2

Joshua WolanykErick Dykstra

Chamika HippolaAlma Vela-Ramirez

Lab instructor – Liang Zhang

1. Overview

The goal of this week experiment was to grow a 0.25 μm. silicon oxide layer on a series of silicon wafers. This was accomplished by first cleaning the silicon wafers to have a bare surface. After the cleaning, the wafers were put into a furnace to grow the oxide layer using a wet oxidation process. When the wafers reach the appropriate temperature, steam is introduced into the furnace to oxidize the silicon. When the oxidation is finished, the steam stops flowing, the temperature is reduced, and the furnace is pumped with dry nitrogen. To ensure the desired layer has been oxidized, a wafer map was created, by measuring the oxide thickness at different points on the wafer surface.

2. Starting wafers Ten wafers are used in this experiment, four of them will be test wafers. The wafers have a resistivity of 3.813 Ω/m. They are n-type wafers with a doping density of 1.18* 1015 atom/cm3. The wafers have an average thickness of 380 μm, which is calculated using 4-point probe resistivity measurements.

3. Standard Clean

The standard clean is used to create a bare surface of silicon on the wafer. The purpose of cleaning the wafer is so that the oxide growth does not have impurities in it, and that it grows uniformly across the entire wafer. For the standard clean, two tubs are prepared with water diluted solutions. One has diluted ammonium hydroxide at a ratio of 5:1 and the other tub has hydrochloric acid with a ratio of 6:1. The two tubs are then heated to 80 °C. When the ammonium hydroxide reaches 75 °C, hydrogen peroxide is added in equal ratio to ammonium hydroxide. After the tub is at 75 °C, the wafers are lowered into the ammonium hydroxide solution for fifteen minutes. When five minutes are left, hydrogen peroxide is added in equal ratio with hydrochloric acid to the acid’s tub. When this wash is done, the wafers are rinsed for three minutes. Then the wafers are placed into a dilute hydrofluoric acid solution for fifteen seconds. Then the wafers are rinsed for one minute before they are transferred to the

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hydrochloric acid tub for fifteen minutes. Finally, the wafers are rinsed for three minutes before they are put into a spin rinse.

4. Wet Oxidation

The wet oxidation is done immediately after the standard clean is finished on the wafers. The wafers are placed into a large furnace on a quartz rack. The furnace is set at 800 °C with a flow of 1.0 slpm dry nitrogen for the ambient temperature. The wafers are slowly loaded at a rate of an inch per minute into the furnace, so the temperature change is not too fast and damages the wafer. After the wafer is loaded, the temperature is set to 1100 °C to have a quick oxidation, and the bubbler that introduces steam is set to 98 °C. After the temperature reaches the set temperature, the steam is introduced into the furnace to begin oxidation. The steam is introduced with nitrogen at a rate of 200 sccm. The oxidation takes 12 minutes and 16 seconds, and the steam is turned off after. After the wet oxidation, the furnace is set 600 °C with a flow of 0.3 slpm dry nitrogen.

5. Results – Wafer MapThe film thickness is about 50% thicker than was estimated. The average thickness was about 369 nm. The filmetric uses the reflection percent of different wavelengths to determine the thickness of the silicon dioxide.

Figure 1. Wafer Map

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5. Appendix - Operating conditions

The oxidation temperature used was 1100 °C, or 1373 K. Using the Boltzmann constant, the value of kT is 118.3 meV. The target oxide thickness is 0.25 μm and the initial thickness is 0 μm, because the wafer is bare. These values can then be plugged into three equations, which are presented below. The linear and parabolic coefficients are presented in Equations 1 and 2, whereas the oxidation time is presented in Equation 3.

Bwet=(386 μm2/hr )e−0.78ev /kT (1)

( BA

)wet

=(9.70∗1 07μm /hr )e−2.05eV /kT (2)

OxidationTime t= t2oxB

+ t ox(B/ A)

−τ (3)

Plugging our operating conditions into these equations, the calculated wet oxidation time for our wafers was 12 min and 16 s.

The resistivity was measured through a four-probe technique. The measurements were taken at the middle and then at four evenly spaced points around the outside. These measurements were then plugged into Equation 4 presented below:

R s=π

ln (2)R (4)

Where: R is the resistance that was measured, in ohms, Ω.Rs is the sheet resistance of the wafer, in ohms, Ω.

.Then the sheet resistance was plugged into

ρ=t RS

Where: t is the distance between the wires measuring the voltage (cm) ρis the resistivity of silicon (Ω/m)

Using these equations, the wafers’ average resistivity was measured to be 3.813 Ω/m.

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