Solid state lighting (SSL) products are now widely available in the market after two decades of research and development in the field of GaN-based LEDs.
SSL is considered as a “green” product, as it has the potential to reduce the global energy consumption for lighting purposes. However, in addition to the sustainability factor the entire value chain also has to be taken into account. Therefore, it is important to thoroughly assess each stage of the production process, thus making an end product truly energy-efficient and resource-friendly. This article focuses on AIXTRON’s CRIUS® II-XL reactor and demonstrates to what extent this platform can play a role in a resource and eco-friendly production.
Metal organic chemical vapor deposition (MOCVD) is a key manufacturing step used for developing white LED chips. The process is performed at high temperatures for a number of hours, which means using a significant amount of electricity to power the electrical heating system. Components that are not actively heated have to be cooled which requires a large amount of cooling water.
The LEDs are composed of GaN layers. Pure elements such as Indium and Gallium are available only in small quantities. Therefore. the manufacturing of these elements is quite difficult and leads to relatively high cost. Thus, it's viable to use these materials in the MOCVD process. Also, large amounts of carrier gases such as hydrogen are used, but these gases have to be produced and purified which again consumes significant amounts of energy.
Three Approaches to Save Resources and Cut Manufacturing Cost
The main indicators for any resource and cost efficient epi process are:
- Precursor material consumption per wafer
- Electricity consumption per wafer
- Carrier gas consumption per wafer
- Cooling water consumption per wafer
Three methods have been identified to save the resources mentioned above. These include:
Usage of certain resources can be considerably reduced by increasing the MOCVD reactor to a larger size. This is also applicable for cooling water and electricity.
The electrical power needed to sustain an MOCVD reactor at a standard Gallium Nitride growth temperature does not accurately scale with the reactor size. This means, while the total power consumption per growth run is greater for a larger reactor, the amount of electricity used per single wafer is relatively less. Since most of the generated heat is dissipated into the cooling water, a similar consideration is relevant for the water consumption. MOCVD systems are supplied with cooling water from a closed loop and hence the water is re-used. However, water circulation and cool down requires electrical energy which has to be considered when measuring the energy consumption of the reactor.
Figure 1 shows how electrical power consumption is saved by scaleup, demonstrating the electricity consumption of different CCS reactors. CRIUS®, CRIUS® II, and CRIUS® II-L are a range of reactor up-scales, which lead to reduced power consumption.
Figure 1. Electrical power consumption per 4” wafer for Close Coupled Showerhead® reactors.
MOCVD reactors from AIXTRON have the highest utilization rates for MO gases and precursors. The unique Close Coupled Showerhead® design in the CRIUS® reactors enables highest precursor utilization. This utilization is available on the entire wafer susceptor or carrier. Therefore, the way substrates are assembled on the susceptor, or which area fill factor is realized, defines the on-wafer utilization efficiency for gases and MOs. Moreover, the showerhead can be precisely matched to the active susceptor area. For instance, the transition from CRIUS® II-L to CRIUS® II-XL is a good example of such geometrical optimization. CRIUS® II-XL uses the same susceptor diameter as CRIUS® II-L, but with optimum matching of showerhead and growth areas and maximized fill factor. Therefore, significant improvements in water and electricity consumption per wafer can be realized, with reduced TMGa consumption per wafer (Figure 2).
Figure 2. Trimethylgallium (TMGa) consumption per 4” wafer for Close Coupled Showerhead® reactors.
Process Cycle Time Reduction
Resource consumption of an MOCVD tool can be reduced by lowering the cycle time. The total cycle time comprises growth time and non- growth time. Although the non-growth time has been censurable reduced, the majority of LED manufacturers have not leveraged the possibility of reduced growth time.
AIXTRON has addressed this issue in two ways; the first is to apply an active top side temperature control (TTC) in the CRIUS® II-XL solutions and the second is to introduce a high growth rate process without affecting the efficiency of MO and gases.
Top side Temperature Control (TTC) : An “ARGUS” pyrometer array is used by the TTC setup to read the exact surface temperature of the wafer carrier. After processing this temperature reading, it is fed back into the control of the three heater zones that heat the wafer carrier, as shown in Figure 3. This principle enables precise adjustment of temperatures and also reduces the settling times needed after altering the growth temperature.
Figure 3. Schematic of the TTC (Top side Temperature Control) in a CRIUS® II-XL MOCVD reactor.
High growth rates: One way to achieve short cycle times is to combine higher growth rates with improved utilization efficiency of the MO precursors. However, existing MOCVD tools cannot achieve this due to their low MO efficiencies. To increase growth rates, the quantity of TMGa fed into the reactor is simply increased. This works for most MOCVD reactors enabling growth rates of 2 to 3µm/h. (Figure 4, lower left part). However, after a specific threshold is surpassed, these MOCVD reactors will not increase growth rates significantly, even when large amounts of TMGa are utilized. This means, additional material is simply wasted.
Figure 4. Growth rate of CRIUS® II-XL and other MOCVD reactors as a function of the TMGa flow
Featuring a unique design, AIXTRON reactors are not affected from these issues. The MO utilization efficiency of these reactors is better, even in the low growth rate regime. They also help in adjusting the growth rates beyond 30µm/h without affecting the MO efficiency (Figure 4, upper right part).
A unique advantage can be created by integrating fast temperature control and high growth rates. This reduces cycle time and saves resources and manufacturing costs. The reduction of hydrogen consumption in CRIUS® reactors is shown in Figure 5. In Figure 6, cost for water, electricity, MOs and gases have been factored to quantify the exact cost.
Figure 5. Hydrogen consumption per 4” wafer for Close Coupled Showerhead® reactors.
Figure 6. Total resource consumption cost per 4” wafer for Close Coupled Showerhead® reactors.
MOCVD tools can enable a more efficient utilization of resources, such as electricity, water, gases and metal organics. The CRIUS® II-XL platform from AIXTRON is a complete range of resource-saving technologies. A combination of fast temperature control and high growth rate boosts the overall resource efficiency of the CRIUS® II-XL reactors.
About Aixtron AG
AIXTRON AG is a leading provider of deposition equipment to the semiconductor industry. The Company's technology solutions are used by a diverse range of customers worldwide to build advanced components for electronic and opto-electronic applications based on compound, silicon, or organic semiconductor materials and more recently carbon nanostructures.
Such components are used in display technology, signal and lighting technology, fiber communication networks, wireless and cell telephony applications, optical and electronic data storage, computer technology as well as a wide range of other high-tech applications.
This information has been sourced, reviewed and adapted from materials provided by Aixtron AG.
For more information on this source, please visit Aixtron AG.