Thin film deposition is one of the three core steps in wafer manufacturing, and the technical parameters of the thin film directly affect chip performance. The continuous shrinkage of semiconductor devices has put forward higher requirements for thin film deposition processes, and ALD technology has demonstrated advantages in advanced semiconductor process applications due to its highly controllable deposition film thickness, excellent uniformity, and three-dimensional shape preservation.
Due to inherent advantages such as low-temperature deposition, thin film purity, and excellent coverage, ALD (Atomic Layer Deposition) technology has been applied in semiconductor processing and manufacturing since the early 21st century. The high-k dielectric deposition of DRAM capacitors was the first to adopt this technology, but recently ALD has also developed increasingly widespread applications in other semiconductor process fields.
The techniques used for thin film deposition include physical vapor deposition (PVD), chemical vapor deposition (CVD), and atomic layer deposition (ALD).
ALD is a variant of CVD, which is a method of depositing substances layer by layer on the surface of a substrate in the form of a single atomic film. The substrate is sequentially exposed to two active gas-phase precursor materials to form ALD compound materials. The substrate is only exposed to one precursor at a specific time, and the exposure time is controlled to be very short, so that only a sub single atomic coating layer of the adsorbed precursor is formed on the substrate. Accurate control of film thickness can be achieved by controlling the number of sedimentation cycles. Capable of depositing ultra-thin films at the nanoscale.
At present, ALD technology can be subdivided into TALD, PEALD, SALD, etc. The types of thin films prepared include oxides, nitrogen (carbon) compounds, metal and non-metal elements, covering dielectric layers, conductors, and semiconductors. The self confinement and wide window temperature characteristics of ALD reaction make the grown thin films have good step coverage, large-area uniformity, density without pores, and easy precise control of deposition parameters such as thickness. ALD technology is particularly suitable for thin film deposition on complex morphology and high aspect ratio trench surfaces, and is widely used in advanced semiconductor processes such as High-K gate dielectric layers, metal gates, and copper diffusion barriers.
An atomic layer deposition cycle can be divided into the following stages:
1. Introduce a precursor and undergo adsorption or chemical reaction with the substrate surface;
2. Inert gas flushing residual gas;
3. Introduce the second precursor gas and react with the precursor product of the first layer
4. Inert gas flushing residual gas
The key role of ALD technology in semiconductor manufacturing is as follows:
Transistor gate dielectric layer (high dielectric constant)
The dielectric constant describes the ability of a material to retain charge, with higher K values resulting in better storage of charge.
High K-value materials can reduce leakage current at the same capacitance density.
Metal gate
Using metal instead of polycrystalline silicon as the gate material for devices, while metal gates have extremely high electron density, which can shield the vibration of polar molecules, provide mobility within the device channel, and effectively solve the problem of depletion of polycrystalline silicon gates.
Metal replaces the gate and is deposited in the trenches of polycrystalline silicon, requiring a deposition process with good step coverage.
Copper interconnect barrier layer
The commonly used processes for interconnect technology are Al process and Cu process. Cu has better conductivity and can be deposited at low temperatures, making it more widely used. The biggest disadvantage of Cu is its fast diffusion speed, which makes it easy to move inside the dielectric and "poison" the device. Therefore, before copper plating, a layer of anti diffusion barrier must be deposited first ALD technology is used to deposit Cu diffusion layers, and under high aspect ratios, the thin film still exhibits good uniformity and anti diffusion barrier properties.
Microcapacitors
The application of ALD in capacitors mainly includes DAMs below 100nm and embedded DRAM. As the storage capacity expands, the number of internal capacitors increases dramatically, and the size of a single capacitor further decreases. The aspect ratio of the internal grooves of the capacitor becomes larger, and the effective area of the deposited film is about 20 times that of the device itself. ALD technology can meet the requirements of large-area uniformity, high step coverage, and precise control of film thickness.
The application of ALD in the semiconductor field. With the continuous evolution of Moore's Law, the feature size and etching grooves of integrated circuits are constantly shrinking. The increasingly small etching grooves pose severe challenges to the coating technology of grooves and their sidewalls. Traditional PVD and CVD processes can no longer meet the requirements of good step coverage under small line widths. ALD technology has excellent shape retention, uniformity, and high step coverage, and is playing an increasingly important role in the semiconductor industry.
