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RCA Clean Manufacturing Process
Particles, metallic impurities, organic contaminants, a naturally occurring oxide layer, and the micro-roughness of the wafer surface are the five major contaminant types in the semiconductor wafer manufacturing: (Figure 1)
| Contaminants | Source |
|---|---|
| 1.Fine particles | The particles, in general, result from ultrapure water, gas, and chemicals used in the manufacturing process; but they can also come from machines, wafer boats, and even production line personnel. Particles stick to wafer surface or fall into tiny grooves on wafer surface due to static electricity, van der Waals force, capillarity, or chemical bonding |
| 2.Metallic impurities | Metallic impurities mainly come from the collision of ions with inner walls of machines during ion implantation, dry etching and photoresist ashing. Pollution may also come from the manufacturing environment, chemicals, and chemical containers |
| 3.Organic matter | Organic contaminants are carbonaceous compounds. They mostly come from photoresist residues, but they can also come from wall paint, motor oil on the pump, plastic containers, and operation personnel’s body and clothing. You can remove organic contaminants by using solvents such as acetone, ethanol, and TCE |
| 4.Naturally occurring oxide layer | The layer is formed when wafer surface is exposed to oxygen in air or water. Hydrogen bonds (Si-H) on the wafer surface oxidize to form hydroxyl groups (Si-OH), or the silicon is oxidized to form SiO2. Reaction rate is related to concentration of dissolved oxygen and soaking time. |
| 5.Surface roughness | Generally, surface roughness comes from SC-1 clean manufacturing. It is correlated to the mixing ratio of ammonia and hydrogen peroxide, temperature in manufacturing process, and cleaning time |
Figure 1、Five major contaminants in the semiconductor wafer manufacturing process (Source: GPTC WBD
The wet chemical cleaning and etching processes most commonly used in GPTC wet bench equipment are:
1. Wet chemical cleaning process
Chemicals often used include SC-1 (APM), SC-2 (HPM), SPM, HF, and BHF. The composition and function of these chemicals are listed in the table below. The US company RCA invented the SC1 and SC2 cleaning processes, so they are also called RCA-1 and RCA-2.| Cleaning Chemicals | Method |
|---|---|
| SC-1(APM) | The ammonia solution is slight alkaline. It is used to clean the surface by inducing mutual repulsion between the wafer surface and fine particles. Hydrogen peroxide can also oxidize the surface of the silicon wafer, and ammonia solution micro-etches SiO2 to remove fine particles. Generally, the APM process is performed with a volume ratio of NH4OH:H2O2:H2O=0.05–1:1:5 at 70 °C. Ammonia solution has a lower boiling point and the APM process tends to cause surface micro-roughness, so to maximize yields, you have to control the concentration ratio of the ammonia solution (NH4OH) and the hydrogen peroxide (H2O2) |
| SC-2(HPM) | This process is important for the removal of metallic impurities. Metal chloride dissolves readily in water, so the HPM process uses hydrogen peroxide to oxidize contaminated metal. Then, the process dissolves the soluble chloride formed in the reaction between hydrochloric acid and metal ions. Generally, the HPM process is performed with a volume ratio of HCl: H2O2: H2O = 1:1:6 at 70 °C. It takes 5–10 minutes of cleaning |
| SPM | This process removes organic matter on wafer surfaces. It uses Caro's acid generated from sulfuric acid and hydrogen peroxide. The acid has strongly oxidizing and dehydrating power. It destroys the hydrocarbon bonds of organic matter to remove organic impurities. Generally, the SPM process is performed with a volume ratio of H2SO4:H2O2=2-4: 1 at 130 °C. It takes 10–15 minutes |
| HF(BOE) | This process removes naturally occurring oxide layers from silicon wafer surfaces. It uses dilute hydrofluoric acid (0.49–2%) or a buffer solution. The buffer solution is generated from hydrofluoric acid and ammonium fluoride (HF/NH4F=1:200-400). The process takes 15–30 seconds at room temperature |
2. Wet etching process
The advantages of the wet etching technique are characterized by its streamlined manufacturing process, low cost, high-performance etching selectivity, and fast yield rate. The wet etching mechanism is as follows:1.An oxidizing agent is normally used to oxidize the etching material. A suitable acid is then used to dissolve the oxidized material.
2.A surfactant and buffer solution are often added to the etchant to maintain stability; this will stabilize the rate of etching and reduce chemical consumption.
Common wet etching processes are described below:
| Type of wet etching | Method |
|---|---|
| Silicon dioxide layer etching (SiO2 Etching) | A buffer solution of hydrofluoric acid and ammonium fluoride (HF/NH4F; BOE or BHF) is used to etch the silicon dioxide layer and initiate the following chemical reaction: SiO2 + 4HF + 2NH4F ® (NH4)2SiF6 + 2H2O When using hydrofluoric acid to etch silicon dioxide layer, it is the concentration of [HF2-] that determines the etching rate: if the HF concentration is kept constant and the NH4F buffer solution provides a large amount of F- ions, the etching rate will remain stable. At the same time, typically a small amount of surfactant is added to a buffer solution in order to assist the chemical exposure of hydrophobic silicon wafer surface. This will make the reaction more uniform and improve the wettability of the wafer, thus also preventing roughness on the etched wafer surface. Factors affecting the rate of etching include: (1) SiO2 layer patterns: Looser structures (such as naturally occurring oxide layers: Native Oxide) produce faster etching rates (2) Reaction temperature: Higher temperatures produce faster etching rates (3) Buffer solution mixing ratio: Higher ratios of HF produce faster etching rates |
| Silicon layer etching (Silicon Etching) |
Most current manufacturing processes etch silicon with a mixed solution of nitric (HNO3), hydrofluoric (HF), and acetic acids (CH3COOH). The process comprises two chemical reactions: Si + 4HNO3 ® SiO2 + 2H2O + 4NO2 SiO2 + 6HF ® H2SiF6 + 2H2O The strongly oxidizing HNO3 is used to oxidize silicon into SiO2. HF and SiO2 then react to generate a soluble silicon fluoride acid. CH3COOH acts like a buffer solution by keeping the etching rate stable. The ratio of nitric acid and hydrofluoric acid is key to etching concentration |
| Silicon nitride layer etching (Silicon Nitride Etching) | Generally performed using 85% phosphoric acid at 160–170 °C to etch the Si3N4 layer. The chemical reaction is as follows: Si3N4 + 4H3PO4 + 10H2O ®Si3O2 (OH)8 + 4NH4H2PO4 It is worth mentioning that the etching selection ratio of hot phosphoric acid to silicon nitride and silicon dioxide is greater than 20:1, and the etching rate is about 60 angstroms/minute |