Semiconductor Manufacture Flashcards
MOS Transistor
Threshold Voltage in Transistors:
In a metal-oxide-semiconductor field-effect transistor (MOSFET), the threshold voltage (Vth) is the minimum gate-to-source voltage required to turn the transistor on and allow current to flow from the source to the drain.
When the gate voltage is less than the threshold, the transistor is in the off state, and reducing the voltage by 60 mV can help in reducing the leakage current.
Leakage Current:
Leakage current refers to the small amount of current that flows through a transistor even when it’s in the off state.
To minimise leakage current, especially when the gate voltage is less than the threshold, reducing the voltage by a small amount (e.g., 60 mV) can be an effective strategy.
Voltage Scaling:
Voltage scaling is a technique used to reduce power consumption in integrated circuits by lowering the operating voltage.
The energy consumed by a circuit is proportional to the square of the supply voltage (E ~ CV^2), where E is energy, C is the capacitance, and V is the voltage.
By reducing the operating voltage (V), the energy consumption of the circuit can be significantly decreased.
Device Scaling
High-Stress Film to Impart Tensile Strain
Tensile strain in the channel region of a transistor can help enhance carrier mobility, which is the ability of charge carriers (electrons or holes) to move through a material.
By adding a high-stress film, which imparts tensile strain, the effective mass of electrons in the channel may be reduced, leading to increased carrier mobility.
Enhanced carrier mobility can result in improved transistor performance, including faster switching speeds and reduced threshold voltage.
Adding SiGe (Silicon-Germanium) to Add Compression:
Silicon-Germanium (SiGe) is a semiconductor alloy that can be used to modify the properties of silicon in a transistor.
When SiGe is incorporated into the transistor structure, it can introduce compressive strain in the channel region.
Compressive strain can increase carrier mobility for holes (positively charged carriers), leading to improved transistor performance.
Similar to tensile strain, compressive strain helps in reducing the threshold voltage and enhancing overall device characteristics.
Hi-K Metal Gate:
A High-K (Hi-K) dielectric material is used as the gate insulator in modern transistors instead of traditional silicon dioxide. This helps to overcome the limitations of the silicon dioxide insulator at smaller technology nodes.
Hi-K materials have a higher dielectric constant, allowing for better gate control over the channel. This facilitates the reduction of gate leakage current and enables further scaling of transistor dimensions without significant loss of control.
3D Transistors:
Traditional transistors are planar structures
In 3D transistors, the channel is a vertical fin, and the gate surrounds the fin from multiple sides. This design provides better control over the channel, reduces leakage current, and enhances overall transistor performance. It also allows for more efficient scaling of transistor dimensions.
Key Manufactruing Steps
Deposition
Physical vapour deposition: involves the transfer of material from a source (solid or liquid) to a substrate in the form of vapor. This vapour is then condensed on the substrate, forming a thin film. PVD does not involve chemical reactions; it relies on physical processes such as evaporation or sputtering.
PVD: very good coverage on horizontal surfaces but less so on the vertical
Plasma-enhanced chemical vapour deposition: combines the principles of chemical vapour deposition (CVD) with the use of a plasma to enhance the deposition process. It involves introducing reactive gases into a vacuum chamber, where plasma is created. The plasma activates the chemical reactions, leading to the deposition of a thin film on the substrate.
CVD: improved coverage on the vertical but it is very non-uniform
ALD: very good
Etch
Capacity Coupled Plasma: involves creating a plasma by applying radiofrequency (RF) power to the gas in a low-pressure environment. The RF power is applied using electrodes, and the resulting plasma is used for material removal in etching.
Inductively Coupled Plasma: using a coil to generate a high-frequency electromagnetic field, which in turn creates a plasma. ICP is known for its ability to achieve higher plasma densities compared to CCP.
more controlled and directional etching.
Well-suited for applications requiring high precision
Lithography - EUV
EUV lithography utilises extreme ultraviolet light with a wavelength in the range of 13.5 nanometres (nm). This is significantly shorter than the wavelengths used in traditional optical lithography (which uses deep ultraviolet light), allowing for much finer resolution.
Future Candidates
Transition Metal Dichalcogenides, SD
Monolayer channel thickness so better gate control
Better mobility than Silicon at scaled channel thickness
High effective mass, larger band gap so low SD tunnelling
FerroelectricFET: they can switch between polarisation states in response to an external electric field. The polarization can be reversed by applying an electric field in the opposite direction.
Magneto Electric Spin-Orbit
