Chemical Vapor Deposition (CVD)

So far, only the growth of silica layer using high-temperature furnace tubes has been discussed. As for other thin film materials such as polysilicon, silicon nitride, tungsten or copper, how do they grow and stack onto silicon wafers?

Basically, high-temperature furnace tubes are still used, but due to different chemical deposition processes, there are different working temperatures, pressures, and reaction gases, collectively known as chemical vapor deposition.

According to the research and development process of chemical vapor deposition, briefly introduce "atmospheric pressure chemical vapor deposition", "low-pressure chemical vapor deposition", and "plasma assisted chemical vapor deposition":

1. Atmospheric Pressure CVD (APCVD)

The earliest developed CVD system, as the name suggests, operates in an atmospheric pressure environment and has a similar appearance to an oxidation furnace tube. The chemical vapor of the material to be grown flows uniformly from the upstream of the furnace tube towards the silicon crystal. As for why it will deposit on the surface of the silicon crystal, a qualitative explanation can be provided using the boundary layer theory:

When viscous chemical vapor blows horizontally over the silicon chip, the silicon chip, like the furnace tube wall, is a solid boundary. Due to the significant change in velocity within the boundary layer about 1mm near the chip surface (from the vapor velocity at the outer edge of the boundary layer to the zero velocity on the chip surface), a dragging external force is applied to drag the chemical vapor molecules; At the same time, due to the surface temperature of the silicon chip being higher than the vapor temperature at the outer edge of the boundary layer, the chip will release heat to supply the energy required for the dragged chemical vapor molecules to dissociate and precipitate the thin film material on the chip surface. So basically, chemical vapor deposition is the application of natural transport phenomena.

The atmospheric pressure chemical vapor deposition rate is quite fast, but the texture of the growing film is relatively loose. In addition, if the wafer is not placed horizontally (which takes up too much space), the thickness uniformity of the film will be poor.

2. Low Pressure Chemical Vapor Deposition (LPCVD)

In order to carry out mass production of 50 or more wafers, the wafers inside the furnace tube must be vertically and densely placed on the crystal boat, which clearly leads to the problem of thickness uniformity of the deposited thin film; Because the assumption of the boundary layer problem on a flat plate is no longer appropriate, the chemical vapor immediately enters a separation state in the viscous flow field after passing through the first wafer, and the reverse pressure gradient will bring the downstream chemical vapor back upstream, creating a chaotic atmosphere.

Reducing the environmental pressure of chemical vapor is a feasible way to solve thickness uniformity, as it is inevitable to place the wafer vertically on the wafer boat. According to the Reynolds number observation of defining viscous flow characteristics, the dynamic viscosity coefficient ν As the pressure decreases, the Reynolds number increases sharply, causing the chemical vapor flow to transition from laminar flow to turbulent flow. It is interesting that turbulence is not easy to separate. In other words, it is an orderly flow in a chaotic environment. Although chemical vapor becomes thinner, slowing down the deposition rate, it still does not separate and flow against the flow after passing through dozens of heavy wafers, maintaining the advantages of uniform thickness and even dense texture. For LPCVD silicon nitride thin films grown at 800oC and 1 Torr, their texture is extremely hard and wear-resistant, and they are also very suitable for etching masks (deposition rate is about 20 minutes, 0.1 micrometers thick)

3. Plasma Enhanced CVD (PECVD)

Although LPCVD has solved the problem of uniform thickness, the temperature is still too high and the deposition rate is not fast enough. In order to lower the deposition temperature first, it is necessary to find another energy source for chemical deposition. Due to the necessity of low pressure for thickness uniformity, the development of plasma energy assistance in low pressure environments (plasma can only exist at 10-0.001 Torr) precisely compensates for the problem of insufficient energy supply in low temperature environments, and even the energy effect of the assistance plasma is higher than that of temperature, resulting in a deposition rate higher than LPCVD. For the PECVD silicon nitride film grown at 350oC and 1 Torr, its wear-resistant texture is suitable for the use of the protection layer before the final cutting and packaging of IC (deposition speed is about 5 minutes, 0.1 micron thick)

The operating principles of PECVD and RIE machines are very similar. The former uses plasma to assist deposition, while the latter uses plasma to perform etching. The difference lies in the use of different plasma gas sources, and the working pressure and temperature are also different.

Huaqi Technology's main products include oxidation diffusion furnace series, chemical vapor deposition (LPCVD system, PECVD system), epitaxial and crystal growth (HVPE, MOCVD and VB, VGF, etc.), RTP rapid heat treatment, gas cylinder cabinet/special gas cabinet/gas source cabinet, semiconductor tail gas (waste) treatment, vacuum heat treatment furnace (double chamber gas quenching, single chamber, traditional tube furnace, etc.), chain furnace series, diffusion furnace body series, etc.