Advances in Waste Gas Treatment Technology in Semiconductor, PV and LED
Waste Gas Treatment is an often overlooked but essential component of manufacturing processes. Historically several serious accidents have occurred in factories where no abatement was installed, but still today gas treatment is often looked at, as a cost rather than safety issue.
Reasons for industrial Waste Gas Treatment
There are three main reasons why gas treatment is installed in factory:
- The first is health and safety, many of the process gases used are flammable or explosive such as H2 or SiH4 while others are poisons such as COF2, HBr, F2 and so must be treated before coming into contact with people or wildlife.
- The second reason is that many gases used are dangerous for the environment such as CF4 which has a lifetime of approx. 50.000 years in the atmosphere and has a Global Warming Potential of 6.500 times that of CO2.
- The third reason for gas abatement is that a lot of waste gases contain a lot of particles or dust in the gas flow. For example this dust is often created during Silane (SiH4) based deposition processes, the reacted Silane produces a Silicon Dioxide as a very fine dust as a waste product. This dust has the possibility to disrupt the facility exhaust leading to unintended downtime in the manufacturing plant. What makes the dust difficult to treat is that the particle size is often submicron and easily passes through wet scrubbers or physical filters without being trapped in the scrubbing liquid.
The provision of gas abatement is therefore an important part of the fab facilities. Typically gas treatment tools are situated in the sub fab, and placed after the vacuum pumps. There are very few atmospheric processes in use, with vacuum based processes being in the majority. Best practice installation results in the gas abatement tool tool being placed as close to the pumps as possible. In turn, the pumps are also placed as close to the process tool as possible.
This concept is known as Point of Use(PoU) and is typical in the high tech industries. The physical closeness to the vacuum pumps results in several advantages, a shorter gas line between the two tools with less chance leaks and a cost saving potential as some processes require this line to be heated to avoid condensation within the waste gas line.
The basic concept of waste gas treatment tools is the same although several different technologies are available. Most tools use two stages in treating the waste gas flows, however depending on the process gases, one stage maybe more than adequate, be it a dry burner or a wet scrubber. In two stage systems the waste gases are broken down thermally to their constituent elements, oxidised to form a stable by product before being washed in a scrubber to remove any acidic components. The first stage of decomposition can be performed by several differing technologies such as ‘Burn’, ‘thermal’ and ‘plasma’. The last two use electricity as an energy source while burn technologies use natural gas, LPG, or H2 as a fuel source.
The function of the scrubber is to transfer the remaining hazardous gases into the scrubbing liquid where they can be more easily treated. Often, lye is used to neutralise the scrubbing liquid prior within the system, this generally results in a lower water consumption and consumable usage. A strongly acidic scrubbing liquid has a large impact on the lifetime of internal components and therefore COO of the tool.
Optimized Waste Gas Treatment with STYRAX
DAS Environmental Expert has recently introduced several new tools onto the market with new technology compared to earlier generations to improve the overall tool performance.
STYRAX, a burn wet system shown in Figure 1, is one of the most flexible gas treatment systems available. It is capable of treating both CVD processes with a large particle load and etch processes with their associated PFC gases. In comparison to the previous generation of tool there are numerous advantages, however here we will concentrate on three.
Improved PFC abatement:
It is clear that the treatment of perfluorinated compounds (PFCs) before discharge to the atmosphere is a critical function of any abatement system. The main issue with many PFCs is that they are extremely stable molecules, the strength of the Carbon-Fluorine bond which is found in PFCs, is the strongest single bond known in organic chemistry, it also strengthens as more Fluorine atoms are added to the Carbon atom.
CF4 is an example of a frequently used PFC gas used in the clean stages of etch processes in semiconductor processing. It is extremely stable and requires large amounts of energy to break the molecule into its constituent elements. The second problem of dealing with the by-products before they recombine is also important, therefore the design of the quench stage is critical to an efficient abatement system.
In order to crack the CF4 molecule temperatures of approximately 1.400°C must be reached. This is attained by a smart reactor design where the waste gases are contained within a reactor that efficiently distributes the heat of combustion and turbulent gas flow ensures a thorough mixing within the chamber. The fuel gas and Oxygen provides both the energy and the oxidant to allow the CF4 disassociate.
CF4 + CH4 + O2 => CO2 + HF + H2O
The HF is washed out of the gas within the scrubber and neutralised either within the abatement tool or in the waste water treatment plant. The particular abatement applications are optimised using liners that fit within the reaction chamber the length and diameter can be optimised for differing applications. By using an optimised liner within the combustion chamber the results shown in figure 2 could be achieved. This figure shows the amount of fuel gas required to treat 0.1slm of CF4 in various flows of N2 and achieve a DRE of >99%. We believe this is excellent performance for an abatement tool.
Reduced NOx emissions:
In CVD processes a commonly use gas is NF3, again in the clean process step. The reaction process can be coarsely described in the following equation.
CH4 + O2 + NF3 => CO2 + H2O + N2 + HF
In the combustion zone the amount of Oxygen is stychometrically regulated to ensure there is no excess oxygen left over from the combustion of the fuel gas. This is achieved using Mass Flow Meters (MFCs) for both the fuel gas and oxygen. In the combustion zone the waste gases break down but are not oxidised in the primary combustion zone but in a cooler secondary zone where additional oxygen is introduced through a multiple liner system. This process is well known in and used in multiple applications and has the advantage of reducing the NOx. This process is shown in figure 3.
Reduced Maintenance Intervals:
All abatement systems suffer from a particle build up within the system, for example SiO2 in Silane based deposition processes. In order to improve the maintenance costs customers are always pushing for longer intervals between cleaning. The existing ESCAPE system requires regular cleaning as deposits fall onto the burner module due to the burning from bottom to top. The design however allows a skilled technician to clean the reactor in approximately 10 minutes. Some customers have requested longer intervals and so the maintenance concept of the STYRAX was reviewed. The burning from top to bottom allows particles to fall away from the burner, the purged inlet design prevents particle build up on the inlets and turbulent flow within the scrubber tank prevents particulates from settling in the tank and causing clogging. Although we do not have extensive field experience on the STYRAX, we have used an identical concept on a larger tool in the Photovoltaic Industry and achieved a 6 month cleaning cycle.
EDC for Fine Dust reduction
A further abatement problem that is often arises is that of particles. Particles are generated in many processes especially Silane and Tungsten deposition based processes. The problem that occurs is that the sizes of the particles are of order of 1µm and thus are very difficult to capture. Typically this particle stream will pass straight through a wet scrubber stage without being captured, clogging the house exhaust. Other commonly used techniques practised involve a physical filter, however, this is also non optimal as the filter clogs regularly and has to be cleaned using compressed air, which just has the effect of distributing the particles around in the surrounding environment, with the possible impact of health issues. DAS addressed this problem by using an electrostatic filter as shown in figure 4.
The key technology used here is a column in which a high voltage electrode generating an electric field is placed. The walls of the column have a film of water flowing down them, this acts as the ground electrode and is used to trap the particles within the water. The high voltage electrode generates ions which stick to dust particles; these charged particles attract other dust particles which are then trapped in the fluid film. The system is self-cleaning, and purges itself on a user defined interval to ensure the electrodes are kept clean. The efficiency of the tool is in excess of 99.9% by mass. The dust particles are then treated in the waste water treatment plant, typically using sedimentation and a filter press. The overall result achieved is that the house exhaust remains free from dust and the necessary maintenance interval is lengthened.