Precursor vaporizer systems are used to evaporate and meter liquid precursors that are fed into a CVD, PECVD or ALD deposition process as a gas phase. The evaporation and metering of the precursors takes place either via a system without a carrier gas, the direct evaporator or using a carrier gas.

Typical applications include the evaporation and metering of water, solvents, silanes, chlorides and metal alkyls such as SiCl₄, TiCl₄, SiHCl₃, HMDSO, DEZ, TMA or TEOS. Naturally, all other precursors can also be used; we are happy to assess the feasibility of specific precursors and operating ranges. The evaporation systems are frequently used in CVD, PECVD or ALD processes for gas-phase deposition, usually at reduced process pressures (LPCVD) or with plasma assistance (PECVD). Applications also arise at atmospheric process pressures, such as in flame hydrolysis (FHD) or plasma spraying processes.

Among the distinctive features of the evaporation systems are, in addition to the ability to evaporate without a carrier gas, the ability to supply multiple consumers from a single evaporator. It is possible both to supply several process chambers simultaneously and to feed precursor vapour into the process chamber at various points.

The ability to supply multiple consumers from a single evaporator reduces the number of components, control elements and the space required for installation compared with other designs. Overall, this helps to reduce the costs of maintenance, plant engineering and system integration. Another option is to integrate two evaporators into a single system with a shared control system.  

Precursor vaporizer Direct evaporation systems (baking)

The precursor is fed into a stainless steel tank, the evaporator, where it is regulated to the desired evaporation temperature. The resulting vapour phase is regulated to the set flow rate via a mass flow controller (MFC) and fed into the process. Additional gas streams can be adjusted and fed into the process, irrespective of the required amount of precursor.

Precursor Vaporizer - Bubbler type

Bubbler systems, on the other hand, utilise a carrier gas flow to deliver the precursor by passing the carrier gas through a reservoir vessel. These systems are simple in design and, when combined with standard containers (e.g. DOCK), provide a cost-effective, reproducible solution within a range of accuracy that is sufficient for many processes. The range of accuracy can be improved by using additional sensors. For example, a humidity sensor can be used in conjunction with a water evaporator to measure the humidity downstream of the bubbler. The required humidity level can be achieved by mixing in dry nitrogen (see AKF humidification stations).

Processes in which accuracy is of the utmost importance should, however, favour direct evaporator systems.

Precursor vaporizer DLI systems (Direct Liquid Injection)

DLI systems represent another variant. Here, the precursor is evaporated in a metered manner using a liquid flow controller and then mixed with the carrier gas. To enable evaporation at the level of the carrier gas pressure, the gas flow itself is utilised for aerosol formation and evaporation. Process temperatures range from room temperature to well over several hundred degrees Celsius. Owing to the independent flow control of the precursor and carrier gas, these systems achieve a high degree of accuracy in absolute dosing, similar to direct evaporators, but can reach higher maximum pressures.

Such equipment is available on the market from various manufacturers, and we at SIGA GmbH integrate it into our systems as required. However, this complex equipment makes the systems more expensive and must be operated using high-quality media, as replacing the components is considerably more costly than replacing a simple MFC.

Precursor vaporizer Charge & Release systems

Charge & Release systems dispense without the use of MFCs, based on pressure and volume in a buffer vessel. The buffer vessel is filled with vapour from the evaporator up to a maximum of the current evaporator pressure and then emptied into the process. The dispensed volume can be controlled via pressure differential, temperature and volume. These systems do not require MFCs and allow for the dispensing of small vapour volumes, as required in ALD processes, as well as the short-term delivery of high flow rates. By adjusting the pressure and temperature, as well as using buffer vessels and outlet orifices, reproducible processes can be set up across a wide range of parameters. Furthermore, the systems operate without being limited, for example, by the temperature resistance of MFCs.

Precursor vaporizer - SIGA GmbH

Siga GmbH is capable of designing and installing all types of evaporator systems. Furthermore, where appropriate, we also design combinations of different operating principles in order to achieve the optimal system or come as close to it as possible.

Our evaporator systems feature metal seals; elastomers are not used within the evaporator system. This fulfils a key requirement for safe operation. The system is designed on a bespoke basis according to the specific requirements of each application. The operating temperature of the evaporator is determined by the process pressure and the vapour pressure of the selected precursor and can, for example, range up to 80°C or up to 140°C.

Pressure and temperature within the evaporator are monitored; in the event of overpressure or overtemperature, the evaporator’s heating is switched off. All components carrying precursor vapour are heated. The system is housed in its own enclosure. Pipe trace heating for the vapour line to the process chamber is also fitted if the line pressure so requires.

The pipework and all components in contact with precursor vapour are made of stainless steel, material no. 1.4404, equivalent to SS316L. Face-seal fittings are used, e.g. Swagelok VCR. The utmost importance is attached to ensuring a leak-tight design. Fixed pipe joints are welded using orbital welding under an inert gas atmosphere.

The evaporator can be automatically refilled whilst the process is running. A precursor refill system is used to refill the evaporator. The precursor is stored in a storage vessel and, with the aid of a gas reservoir, is forced out through an immersion tube and fed into the evaporator.

The evaporator is controlled and monitored – including temperature control and automatic refilling – by the evaporator control system via a programmable logic controller (PLC) with a touch-screen for visualisation and operation. All functions are regulated by this control system. The setpoint for the MFCs and the selection of the desired gas path are determined by the higher-level control system of the coating plant. Communication is via a Profibus DP or another serial interface. The details of the control system, such as the type of operation with options for manual intervention and communication with the higher-level control system, are adapted to the requirements of the process.

The evaporator systems are installed in suitable sheet steel enclosures or in standard control cabinets. The control unit and evaporator can be integrated into a single cabinet or installed in two separate cabinets. The configuration can be adapted so that the system can be integrated into the coating plant as effectively as possible and tailored to the requirements at the installation site.

The integrated PLC control system with touch screen enables automatic regulation of temperature, pressure and level. Continuous replenishment whilst the process is running can be achieved via an external supply system with a gas reservoir – reliably and user-friendly. Communication with the main plant takes place via Profinet, Ethernet with SecureADS, RS485 or other industry standards.

In addition to passive safety elements and enclosed safety components such as safety temperature switches or safety threshold switches, a safety PLC (e.g. TwinSAF) is also used where necessary; this routes safety-critical functions and can interrupt or activate them independently of the actual control programme, based on a logic table derived from a safety assessment.

Safety functions such as shutdown in the event of overpressure or overtemperature are an integral part of the control system. The system can be configured modularly: as a stand-alone solution or as a multi-precursor system with central control. Enclosures made of sheet steel or in explosion-proof designs are available, as is integration into existing control cabinets.