Learn About Thin Film Preparation Technology In One Article

Learn About Thin Film Preparation Technology in One Article

Thin film epitaxial growth is a key material preparation method that is widely used in semiconductor devices, optoelectronics, and nanotechnology.

This process involves the deposition of atoms or molecules of the material layer by layer on the surface of the substrate to form a film with specific properties and structure, so its growth process directly affects the structure of the film and its final properties.

Compared with bulk materials, thin films have the characteristics of easy preparation, easy modification, and low cost. At the same time, thin-film-based devices are smaller in mass and size, and are easier to integrate with Si-based CMOS and Micro-Electro-Mechanical System (MEMS) technologies to achieve high integration.

At present, the technology for preparing thin films mainly includes sputtering deposition, vacuum evaporation, molecular beam epitaxy (MBE), chemical bath deposition (CBD) and other methods.

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Vacuum evaporation method

Vacuum evaporation is a method of heating raw materials (also known as targets) in the evaporator container in a vacuum chamber, sublimating their atoms or molecules to form a vapor stream, transporting them to the surface of a solid substrate with a lower temperature, and then re-condensing and depositing them into a thin film. Vacuum evaporation coating equipment mainly includes vacuum chamber, evaporation source or evaporation heater, substrate, substrate heater and thermometer. Normally, the melting point of the material deposited by thermal evaporation needs to be below 1500°C, and the evaporation rate is adjusted by the amount of heating current during the deposition process. In order to ensure the uniformity of the composition and thickness of the evaporated film and the repeatability of the evaporation process, it is also necessary to additionally equip the substrate rotary table and the quartz partial film thickness monitoring system. Vacuum evaporation coating consists of three main processes, as shown in the figure:

Taking electron beam evaporation as an example, firstly, the solid-phase target is transformed into a vapor phase at high temperature.

Then, the vaporized atoms or molecules are transported between the evaporation source and the substrate, and the number of collisions between the gas-phase particles and the residual gas molecules in the vacuum chamber during flight is directly affected by the vacuum degree and the distance between the target and the substrate, which determines the average free path of the evaporated atoms.The final stage of film formation involves the deposition of vapor-phase particles on the surface of the substrate, which involves key steps such as vapor-phase material condensation, nucleation center formation, nucleation growth, and finally the formation of a continuous film.

Since the substrate temperature is significantly lower than the target temperature, the gas-solid phase particles will undergo a direct gas-solid phase transition on the substrate surface. It is important to emphasize that all of the above process steps must be completed in a high vacuum environment. If the vacuum is insufficient, the evaporated particles will collide frequently with the residual gas molecules, which will not only lead to the contamination of the film layer by impurities to form oxides, but also may be difficult to form a uniform and dense film structure due to the scattering effect of gas molecules, in addition, the target may also be oxidized and burned at high temperatures. Vacuum evaporation has been used to manufacture thin films for decades and is very versatile.

In recent years, in order to inhibit or avoid the chemical reaction between film raw materials and containers at high temperatures, many improvements have been made to crucibles and heating methods, such as: using high melting point heat-resistant boron nitride ceramic crucibles; Using an electron beam or laser as a heating source, a small area of the raw material surface is heated so that the area reaches a high temperature instantly.

In response to the increasing requirements for functional film performance, multi-source co-evaporation and sequential evaporation methods are used to fabricate composite films with complex compositions or multi-layer composite films.

In addition, researchers have developed a reaction evaporation method for compound films that are prone to component segregation during evaporation.

The vacuum evaporation method has the advantages of low cost, simple equipment and easy operation, and the growth mechanism of the film deposited by this method is simple, the film purity is high, the film thickness is precise and controllable, and clear graphics can be obtained by using the mask plate. The main disadvantage of this method is that the kinetic energy of the gas-phase atoms produced by thermal evaporation is lower than that of sputter deposition, and the bond between the substrate and the substrate after re-solidification is weak, which can be improved by heating the substrate.

Sputter deposition method

Sputter deposition technology is an important branch of physical vapor deposition (PVD) technology. It works by using radio frequency energy or laser beams to activate rarefied gases (Ar, O2, N2, etc.) in the vacuum chamber to form high-energy plasma. The ions in these plasmas accelerate the bombardment of the target surface under the action of electric field, and the target atoms obtain sufficient energy to break away from the lattice bondage through kinetic energy transfer, and then migrate in gaseous form and deposit on the surface of the substrate to form a thin film.

The sputter deposition technology currently used mainly includes diode sputtering, tripole sputtering, reactive sputtering and magnetron sputtering, among which magnetron sputtering is the most widely used and most industrialized thin film sputtering deposition technology, and its equipment and principle are shown in the figure.

This technology constructs a closed magnetic field in a vacuum chamber, and its direction parallel to the target surface can confine the plasma and secondary electrons to the area near the target, enhancing the ionization efficiency of argon. This magnetic confinement effect can simultaneously increase the number of high-energy charged particles and their kinetic energy in the plasma, thereby greatly enhancing the bombardment effect of high-energy particles on the surface of the sputtering target and achieving a significant increase in the deposition rate of thin films.

Due to the high film formation rate, atoms do not have enough time to migrate to the lowest energy position in the crystal lattice, so semiconductor films prepared using magnetron sputtering generally have high defect density.

However, this technique can be used to deposit large areas of thin films and can achieve precise control of film thickness through quartz crystal oscillators.

Chemical bath deposition method

The earliest film of lead salt compounds deposited using the CBD method is PbS, dating back to the World War II era. In the sixties of the last century, this technology has been widely used to deposit PbSe films. The schematic diagram of common CBD reactor devices and principles is shown in the figure:

Under certain conditions, the precursor undergoes a hydrolysis reaction to produce Pb2+ and Se2- in the solution, and when the concentration of these two ions increases to exceed the solution concentration product constant, PbSe precipitation will be generated from the solution to form a PbSe film.

The Pb2+ sources are usually Pb(NO3)2 and Pb(CH3COO)2, and the Se2- ion sources are (NH2)2CSe and Na2SeSO3.

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The core technology of CBD technology for depositing thin films is to regulate the hydrolysis reaction of precursors, and control the deposition rate and film formation quality of PbSe films by controlling the concentration of precursors, pH, reaction temperature, reaction time and other process parameters.

CBD process is the mainstream method for preparing PbSe films due to its simple device, fast film formation, low process cost, and easy control of reaction.

In addition, it usually reacts at temperatures below 100°C and is highly compatible with substrate materials.