CVD, or Chemical Vapor Deposition, is a process commonly used to fabricate semiconductor devices. In this method, a solid material is deposited on a substrate by reacting or decomposing one or more precursors in the gas phase.

In your case, you're dealing with CVD selenization, which is a process where a selenium-containing precursor is deposited on a silicon wafer, likely for the production of thin-film photovoltaic devices, such as copper indium gallium selenide (CIGS) solar cells.

The shape and uniformity of the deposited material can be influenced by many factors, including:

• The design and operation of the CVD reactor
• The temperature and pressure conditions
• The flow rates and concentrations of the precursor gases
• The placement of the substrate in the reactor
• The nature and preparation of the substrate

The semi-circular shape you're observing may be due to non-uniform gas flow or temperature distribution in your CVD reactor. It might also be related to the orientation or position of the silicon wafer in the reactor. If the wafer is not positioned correctly or uniformly, this could lead to uneven heating or exposure to the precursor gases, resulting in a non-uniform deposition pattern.

The change in the size of the semi-circle with pressure and deposition time is likely due to changes in the rate and uniformity of the deposition process. Higher pressures or shorter deposition times may lead to less material being deposited and may change the gas flow or temperature distribution, affecting the deposition pattern.

The influence of gaps between the wafers may also be related to changes in the reactor's gas flow or temperature distribution. If the wafers are too close together, this might block or disturb the gas flow, leading to non-uniform deposition.

To improve the uniformity of your deposition, you might need to optimize your CVD process parameters, such as temperature, pressure, gas flow rates, and substrate positioning. You might also need to improve the design or operation of your CVD reactor to ensure a uniform temperature and gas flow distribution. Furthermore, a more detailed study of the process, potentially including computational fluid dynamics simulations, could help to better understand and control the deposition process.

To investigate the deposition pattern, it is necessary to position the substrate at different locations. The distance between the precursor and the substrate also plays a crucial role, along with other process parameters. It is important to adjust the substrate placement and optimize the parameters to achieve the desired results.