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Application Notes
How to stabilize the epitaxial growth process?
Gas delivery for MOCVD
Recent semiconductor process has required atomic scale engineering. Though gases can be controlled precisely by Mass Flow Controllers (MFCs), it is necessary to clarify phenomena for vaporizing liquid or solid precursors. Unstable precursor delivery causes process variation that affects the fabrication yield.
HORIBA offers three-types of vaporizing methods, bubbling, baking and direct injection. Each method has advantages and disadvantages. The bubbling method, among them, is widely used for III-V MOCVD because it’s a simple configuration and can be applied to atmosphere pressure process.
In the ideal bubbling condition, the space region is always at equilibrium with the precursor and the precursor temperature is under stable control. The precursor concentration, C, is described by the ratio of the precursor molar flow rate to the total flow rate. From the equation of the precursor molar flow rate, QP, we can find that keeping C constant is important to obtain stable precursor delivery. There are, however, many factors that make it difficult to stabilize C. For example the bubbler temperature is easily affected by the carrier gas temperature, generation of vaporization heat. The amount of residual precursor also affects the precursor vapor pressure.
Precursor delivery control methods are broadly divided into two: “Concentration control” and “Precursor flow control”. The former is to adjust C by controlling the total pressure, PT, so called “Pressure-based VCC” or by controlling a dilution ratio by using two MFCs, so called “Flow-based VCC”.
HORIBA offers a pressure-based VCC solution, in which a pressure valve and a gas concentration monitor based on a non-dispersive infrared (NDIR) technology are integrated into one module. Concentration control algorism inside the VCC module allows users to just set a concentration and obtain stable precursor delivery.
Because vapor pressure increases exponentially with increasing in temperature, a small change of the bubbler temperature affects the vapor pressure. When the pressure at the downstream from the bubbler is controlled to maintain a constant pressure, the bubbler temperature influences the precursor concentration. For example, an increase in temperature of only 1°C causes about 7% increase in precursor concentration in the downstream line of the bubbler (below right graph). VCC module enables constant precursor concentration control for fluctuating the bubbler temperature. As a result, VCC module contributes a stable process yield.
