Understanding Thermal Oxidation Processes

Thermal oxidation is a procedure in which an oxide layer is grown over the Silicon wafer surface thereby creating silicon dioxide (SiO2). Even though this process is can be applied to any material, but silicon oxidation is the most talked about. Thermal oxidation is performed in a furnace that delivers the temperature needed to raise the oxidizing ambient temperature. Each furnace can accommodate several silicon wafers. This process has the intense capabilities to transform the silicon-based microfabrication process more effectively.

Most assume that depositing silicon dioxide itself is oxidation, but it is to be understood that oxidation is employed merely for altering the present layer of silicon into silicon dioxide and what we do is just modifying which is somewhat analogous to the doping process. Silicon dioxide is a very good insulator with a higher resistivity (> 1 × 1020 Ω cm). Silicon dioxide is resistant to most etching solutions used for metals or silicon. In Material. Silicon dioxide is produced from silicon atoms of the wafer. It does not require chemicals other than purified oxygen and water.

Thermal Oxidation Growth Methods

Dry Oxidation Growth: The silicon wafer reacts with the ambient oxygen forming a layer of silicon dioxide on its surface. The film characteristics include Molecular Oxygen, Slow growth oxide, High Density, and High Breakdown Voltage.

Wet Oxidation Growth: Hydrogen and oxygen gases are introduced where they react to form water molecules, which are then made to enter the reactor where they diffuse toward the wafers. The film characteristics include Water Vapor, Faster growth rates, more porous than wet oxide, and suitable for higher thickness targets

The oxidation level surges substantially with the furnace temperature in both methods. In case of wet or dry, the hundred degrees or more temperature results in the oxidation increases by twice. The principal underlying principle of this remarkable temperature impact on the oxidation percentage is due to temperature reliance on the diffusivity of oxygen and water in blended silicon oxide. The greater the temperature greater is the diffusivity and oxidation capacity.

Another factor that impacts is pressure, either in wet or dry oxidation, the oxidation scale increases significantly with the growth in pressure. The primary benefit of higher-pressure oxidation compared to traditional oxidation is a quicker oxidation rate. High-pressure oxidation offers higher quality and reliability in the overall process. Oxidation-induced stacking faults inadequacies are significantly reduced with higher pressure oxidation, which leads to improved device performance.

Wet oxidation is chosen over dry for cultivating dense oxides since the rate of growth is greater. Nevertheless, rapid oxidation allows additional dangling bonds on the interface providing scope for current to leak along with the interface. The dry method takes a longer time to produce dense oxidation. Wet oxidation as well produces a lesser density oxide, with reduced dielectric power.

Thermal Oxide Specifications

Most used industry-standard Thermal Oxide Specifications include:

• Thickness Range,500Å – 100,000Å
• Thickness Tolerance, Target ±5%
• Within Wafer Uniformity, ±2%
• Wafer to Wafer Uniformity, ≤5% or better
• Sides Processed, Both
• Refractive Index,1.456
• Film Stress, -320MPa ±50 (compressive)
• Wafer Thickness, 100µm – 2,000µm
• Wafer Material, Silicon, Silicon on Insulator, Quartz
• Temperature,800˚C – 1080°C
• Gases Oxygen, Steam
• Equipment, Horizontal Furnace