The reflected power is defined as the portion of the generated microwave power that is not coupled into the process and that returns back to the generator. In microwave processes this is an important parameter employed to assess the efficiency of the microwave energy transfer into the material(s) to be heated or into the plasma. This article describes what the reflected power value means, which parameters influence the reflection of the microwaves and how to minimize the reflected power.
One of the advantages of microwave plasma systems is that, unlike in RF (radio frequency) plasma systems, the reflected power does not cause any direct damage and can be used for process optimization.
In the following example a classical microwave plasma system is described using the model of an atmospheric burner.
Reflected power always occurs when the components of the plasma system (i.e. microwave generator and plasma source) are not properly matched. In microwave processes this occurs due to changes of waveguide geometry, line geometry (coaxial conductor, stripline etc.), but also due to the change of the dielectric load in the process chamber. Thereby the microwave coupling can vary due to changes of the process material, evaporating water, plasma ignition, temperature, pressure etc., which requires an adjustment of the impedance matching in order to decrease the reflected power. The more abruptly these changes occur in the system, the more difficult it becomes to compensate for the mismatch with a single matching element.
All these effects can be explained by impedance matching and visualized by means of a Smith chart. The generator and the load can be assigned (frequency-dependent) complex impedances, which are matched by a complex matching network. The matching network is built by ideally resistive free inductors and capacitors. In the case of Figure 1, this corresponds to the three tuning stubs of the tuner, which can be lowered into the waveguide at different depths.
Due to the existence of the isolator as a protective element, the creation of reflected power does not represent a major issue for the hardware used in microwave applications: the reflected power is absorbed by the water load of the isolator and converted into heat even if the reflected power is 100 % of the power generated by the microwave generator. The reflected power is usually measured at the isolator level and as such, the operator can calculate how much power is absorbed by the load. In general, a microwave process should be operated at the lowest value of the reflected power in order to run the application with maximum energy efficiency.
Due to RF plasma operation at lower frequency by comparison to microwave frequencies (e.g., 13.56 MHz vs. 2.45 GHz), isolators are not available in RF plasma processes and the reflected power becomes critical. High reflected power in the RF process chamber can cause damage to components by unwanted arcing and in the worst case, the reflected power can destroy the generator.
For standard systems, various tuning elements can be used. Besides the already mentioned 3-stub tuner, plate (iris) tuners or E-H tuners can be used. The tuners can be operated manually or automatically to minimise the reflected power by changing the wave transmission behavior from the generator to the load as to optimize the matching.
If a system is very well designed, the wave transmission properties can be predicted. In this case the geometry of the transition can be fixed and does not need additional adjustment and no moving elements will be required.
An example of such a system is shown in Figure 2. The special sequence of the components (magnetron, coupling stage, circulator, transition into the source, power output to the plasma) allows a perfect matching of the impedances, which ensures an almost zero reflected power system.
The reflected power is a process parameter observed in all plasma processes. In microwave processes the existence of the reflected power is, in contrast to RF (radio frequency) processes, a relatively uncritical parameter. The reflected power value should be as low as possible, to optimize the efficiency of the process.
Even small reflections in RF processes can lead to unwanted discharges (arcing) or even to destruction of the generator; in microwave systems the reflected power is a measure for the efficiency of the process. The lower the reflected power, the higher the power available to the process. In microwave processes the reflected power is absorbed by the isolator’s water load and eliminated as heat without endangering any parts of the plasma system.
As deposition is one of the pillars of semiconductor production, it depends on a controlled, repeatable and clean environment. One challenge of the deposition process is, that deposition does not only occur on the substrate but also on the chamber walls. This is a permanent threat of particles and contamination. Hence, the complete and repeatable cleaning of the process-chamber is key for layer quality as critical dimensions keep challenging physical limits.
Microwave plasma is the perfect solution for removing such thick coating off the chamber walls. The advantage of microwave (MW) plasma over RF-plasma makes the difference – chamber walls can be cleaned even without direct expose to the plasma source due to its unique property of microwave.
When it comes to chamber clean, RF-plasma is not ideal due to its anisotropic properties. As ion bombardment is inherent to RF-plasma, it required direct exposure of the parts to be cleaned. In reality, parts of the chamber walls are hidden to the plasma source which causes incomplete clean and subsequent flaking, particles and low yield. Microwave assisted plasma clean is the solution to this problem. The generated radicals reach hidden and protected areas of the chambers which are invisible to the RF-source.
The Muegge remote plasma clean is available for all materials which do not require corrosive chemistry, e.g.:
Semiconductor manufacturing requires repeatable process conditions and clean process chambers. Microwave-plasma allows to clean fast without eroding sensitive components in the chamber (i.e. electrostatic chuck).
It provides the capability for complete removal of dielectrics and organic layers at high speed (>200 µm/h) and high selectivity with no altering of chamber surfaces. The radicals generated produce chemical reactions at the surface, leading to pure chemical etching at high rates with extremely low thermal load.
Microwave assisted plasma is highly environmentally compliant due to nearly complete dissociation of process gases like CF4.
The alternative to conflict diamonds are lab-grown, conflict-free diamonds. Lab-grown diamonds are chemically, physically, optically and visually identical to mined diamonds. In this deep-dive about diamonds, you will learn how microwave energy is able to generate high temperature gas plasma with unique properties for diamond deposition plasma processes – with highly advanced technology from the world’s top supplier and expert in the field of industrial microwave technology: MUEGGE.
For growing real diamonds, the high frequency coupling of microwave energy produces very high plasma density with low sheath potential for an efficient, yet gentle process. The microwave systems produced by MUEGGE are used to generate plasma for man-made diamond applications.
Turn-key CVD diamond systems for both research and production are currently working at 2.45 GHz and 915 MHz frequencies and at powers ranging from 6 to 100 KW.
Industrial microwave systems typically use a variety of standardized or customized waveguide components for delivery of microwave energy, each having a specific and necessary function.
For growing real diamonds, the high frequency coupling of microwave energy produces very high plasma density with low sheath potential for an efficient, yet gentle process. The microwave systems produced by MUEGGE are used to generate plasma for man-made diamond applications. Turn-key CVD diamond systems for both research and production are currently working at 2.45 GHz and 915 MHz frequencies and at powers ranging from 6 to 100 KW. Industrial microwave systems typically use a variety of standardized or customized waveguide components for delivery of microwave energy, each having a specific and necessary function. The individual components are usually available in several different types and waveguide sizes offering different performance characteristics that are well suited for specific heating applications. They can also be arranged in any of several different configurations as required by the application. As a result, the system designer is presented with a wide variety of often difficult choices of components and configurations.
Our experts are happy to assist and consult you regarding the special characteristics you may need for your project. Our engineers know industrial processes and requirements in depth and are able to simulate and develop prototypes with state of the art 3D visualisation tools.
MUEGGE microwave generators typically include a switch-mode power supply. A switched-mode power supply (SMPS) is an electronic power supply that incorporates a switching regulator to convert electrical power very efficiently. This higher power conversion efficiency is an important advantage of the switched-mode power supply and also substantially reduces the overall size and weight of the unit. MUEGGE’s switch-mode power supplies can also be configured with a variety of options making these the most customizable power supply choice and an ideal power source for most industrial microwave heating applications.
Muegge’s MH-Series magnetron heads offer a variety output power and configuration styles. Key features include compact designs, standard waveguide and coaxial output flanges; magnetrons are air or water/air-cooled. The MH-series also offers a variety of options including integrated isolator, reflected HR output power measurement and arc detection.
The excellent output power stability in combination with an extraordinary robustness against effects caused by non-conformal grid instabilities make MUEGGE microwave generators your first choice for long term processes like diamond deposition. MUEGGE offers support for the microwave design and for customer specific developments in the complete range of microwave and microwave diagnostic for diamond deposition processes.
Feel free to contact us for individual consultation, from expert to expert:
Let’s bring power to your project.
They sparkle like mined gemstone, they look like “real” diamonds, and yet, there is one crucial difference:
Lab-grown or man-made diamonds do not bear the risk of supporting inethical actions in regions of the
world where human rights might be violated. A decisive factor – considering that consumers demand
increasing transparency from companies and a so called “blood diamond” case could potentially cause huge
reputation damages. Find out about diamond’s conflict-free twin and the futuristic microwave technology
that makes it grow, shine and thrive.
Conflict or blood diamonds are illegally traded to fund conflict in war-torn regions. The United Nations defines conflict diamonds as “Diamonds that originate from areas controlled by forces or factors opposed to legitimate and internationally recognized governments, and are used to fund military action in opposition to those governments, or in contravention of the decisions of the Security Council.”
The alternative to blood diamonds are lab-grown, conflict-free diamonds. Lab-grown diamonds are
chemically, physically, optically and visually identical to mined diamonds.
Man-made diamonds, also known as engineered or cultured diamonds are grown in highly controlled laboratory environments using advanced technological microwave plasma processes that generate conditions under which diamonds can grow fed from an excited gas phase in a quality as if they were formed in the mantle, beneath the Earth’s crust. These man made diamonds consist of actual carbon atoms arranged in the characteristic diamond crystal structure. Since they are made of the same material as
natural diamonds, they exhibit the same optical and chemical properties and are a great alternative against conflict diamonds.
Diamonds are grown by using a process known as chemical vapor deposition. Tiny diamond fragments, diamond seeds, are placed onto a silicon wafer which is then heated up by a plasma. Plasma is the fourth state of matter and can be thought of as a really hot gas. So hot, in fact, that molecules and atoms are no longer the only species in the gas phase, and instead a soup of ions and electrons, radicals and reactive species whizzing around each other exists.
The plasma, made of hydrogen and carbon, either dissolves the diamond seeds or creates an environment that make them form to grow larger diamonds. If, however, this process goes wrong you can end up with just plain graphite rather than the glittering diamonds you were seeking.
The Chemical Vapor Deposition process, which became viable in the 1980s, does not require high pressure.
A hydrocarbon and hydrogen gas mixture is ionized by microwaves in a vessel at temperatures of about 800 degrees Celsius. This breaks the molecular bonds of the gases, enabling carbon atoms to be deposited on a diamond seed. Growing first slowly into a crystal microscopical layer by layer over days and weeks until the desired size is achieved.
Many of the industrial lab-grown diamonds companies use CVD, which gives the diamond creator greater control over the process. This process control is necessary for production of large volumes of gem-quality diamonds rather than industrial-quality stones for purposes such as cutting machinery.
A leading innovator in microwave and plasma technology, MUEGGE produces systems and components to generate high temperature gas plasma with unique properties for diamond deposition plasma processes.
Read more about our specialized process to produce lab-grown diamonds.
A leading innovator in microwave and plasma technology, MUEGGE produces systems and components to generate high temperature gas plasma with unique properties for diamond deposition plasma processes.
While developing thermoprocessing equipment, testing of prototypes and numerous repetitions cost plenty of
time and resources. 3D simulation capabilities allow you to speed up your prototyping phase, implement
adjustments promptly whilst offering a detailed and comprehensive view to optimize your microwave
application. Learn what you can get out of virtual prototyping tools as we use them at MUEGGE, to power
3D simulation tools cast a realistic and accurate view on your power requirements, applicator dimensions and impedance matchings without having to deal with the production of complex samples. But not every tool and method makes sense for anyone – to save time and achieve best results, consult experts to analyse your microwave material properties and suggest the simulation and analysis variant that fits best to your project goal.
Don’t compromise on the quality of your microwave simulation tools – low-performing server structures or outdated systems corrupt the advantages you get from this technology. By using a high-performance 3D-electromagnetic-analysing software it is possible to carry out jobs such as 3D-designing, EM-analysing or optimizing electromagnetic components within a short time with the utmost precision. Simulation possibilities at MUEGGE, for example, embrace a wide field of applications starting from a simple coupling analysis of waveguide components to a complete simulation of the field strength distribution in complex multimode applications.
Gather comprehensive datasets for scaling processes and expanding the good’s output by coupling individual solver types. These hybrid simulations enable you to analyse and virtualise the complex coaction of high frequency energy and additive electromagnetic influencing systems in an efficient and, at the same time, simple way. This is crucial to determine the efficiency factor of antenna systems or heating performance of resonators.
Thanks to 3D-HF-structure simulations it is possible to reproducibly design frequency microwave filter systems for a low-emission operation of half-open, continuously operating microwave applicators. By a continuous spatial optimization of the choke inlet and outlet structures we are able to consistently guarantee the observation of EMV-threshold values for all applications that are heated by microwaves or by hybrid technologies.
The consequent utilization of the 3D-simulation as a direct process step in your microwave component
development allows compliance with shortest possible development cycles – no need for long test series with complex sample types and numerous repetitions. It offers considerable advantages for a prompt product adjustment at very low development costs. Ask our experts how we can bring simulation power to your project – just mail, call or ping us on LinkedIn.
Fast, efficient, flexible. For many years already the process design in high performance vulcanization is revolutionized by microwave and microwave-hybrid-applications. The advantages are very obvious; you swiftly reach a high vulcanization temperature and hence significantly reduce your exposure time. The precise design of the microwave-antennas, optimized in 3D simulation calculations, can also make your vulcanization processes significantly more efficient. Refresh and deepen your knowledge about this efficient vulcanization technology with an expert’s dive into the winning attributes of microwave applications.
Microwave and microwave-hybrid-applications are perfectly suitable for the heating and vulcanization of synthetic and caoutchouc polymers since these polymer blends absorb electromagnetic waves very well. The typical polymers such as EPDM (Ethylene propylene diene monomer rubber), NBR (Nitrile butadiene rubber) or SBR (Styrene butadiene rubber) absorb over a wide temperature range between 10 °C and 170 °C nearly the complete amount of energy that is provided in the microwave frequency bands around 2.45 GHz and 915 MHz. It is also possible to create application and process variations by a microwave additional heating to reach higher vulcanization temperatures faster and therefore reduce the exposure time significantly.
The design of the microwave-injecting antennas has a crucial influence on the aspired energy distribution in the heating channel. Corresponding to the aim a strong focus on profiles or profile areas is as possible as a homogenous filling of the entire cavity for the processing of highly porous foam and sponge rubber products. For positioning and alignment of the individual antennas of the microwave system are crucial the entire dimensions of the channel cross section as well as the character of walls and transportation system. Using 3D simulation calculation during the design of the heating area we optimize the ideal radiation geometry, alongside the transport direction or across the material cross section as well as the type of the microwave antenna. At the same time it is possible to take in to account of high-density profile areas as for example with PPP-elements (polypara phenylen) or integrated metal inlays via the design of microwave coupling as coaxial antenna system, a slotted waveguide antenna or a horn or fan horn structure.
The antenna system’s supply with microwave energy is realized by serveral, independently adjustable power generators with switch mode power supplies with adjustable microwave power from 1000 W up to 15 kW at 2.45 GHz and 5 kW to 100 kW at 915 MHz. The combination of high-performance generators with more than 6 kW output power and lossless power splitting enable the connection of several antennas to an antenna array and hence to an in-phase energy coupling across wide process tunnel areas. This controlled energy supply permits the construction of heating channels with minimal cross-coupling between the generators and thus an extraordinary utilization of the microwave energy supplied.
No matter whether you require a new highly flexible vulcanization channel with homogenous energy distribution or a low-loss reflector system for efficiency improvement of your existing applicator setup, the microwave technology may offer you an optimal solution. Consult your MUEGGE engineering team for the latest possibilities as well as tailor-made or standard solutions that support your vulcanization project.
Photoresist strip after high dose implant strip remains one of the most demanding processes in
semiconductor industry. The challenge is to remove both the crust (or hardened layer formed during implant) and the soft bulk resist without burning the crust and therefore, without leaving carbonized residues which are very difficult to clean off. Controlled removal of hardened organic material and untouched resist at low temperature is key to cleanliness.
Due to the nature of the RF-plasma, it exposes the substrate to a relative high intake of charged species (ions). These ions transfer kinetic energy when hitting the surface and cause the substrate to heat up. High temperature is very unfavorable to the post high dose implant strip as it will participate to further baking and hardening of the photoresist crust. Cooling of the substrate helps to prevent volume heating-up of the substrate but it cannot avoid local heating of the surface where the actual reaction takes place. Hence, even if the average temperature is kept low, the local temperature remains high
since the underlying reaction (ion-sputtering) is not affected by the cooling. Using Muegge’s microwave assisted plasma systems, the energy transfer is extremely low, no charged particles are formed which could be released into the process chamber. As a result the substrate remains cold, the only heat-source it is the chemical reaction of the exothermic photoresist removal. As the reaction heat is a direct result of the resist-strip process, it can be easily controlled by the process conditions.
Microwave assisted plasma efficiently uses the energy to form highly reactive, neutral particles called radicals. No ions are formed which are the carriers of kinetic energy, causing the substrate to heat up. Therefore the microwave-assisted remote plasma system is ideal for post high dose implant strip where temperature control is critical – no additional polymerization or carbonization occurs and the implanted photoresist can be removed smoothly.
The radicals released from the microwave assisted plasma guarantee a smooth, pure chemical removal of the damaged photoresist. As a consequence, the thermal load is extremely low which guarantees isotropic photoresist removal without further carbonization of the damaged photoresist.
Radical-only formation results in little to no interaction with the surrounding surface of the plasma chamber, the temperature remains low (Figure 1). The pure chemical HDIS-process can be fine-tuned
for the three steps of the removal process:
No ions mean no need for additional cooling of the substrate.
The three steps of high dose implant strip require appropriate flexibility to meet the different demands of each step (Figure 2). Microwave assisted plasma allows to cut through the crust, remove the bulk resist without blistering or resist burning and removed residues softly and with high selectivity during over-ash. Due to the physical properties, microwave assisted plasma is specially designed for fast but smooth ashing without attacking sensitive areas. The radicals generated in the plasma initiate the chemical reaction at the surface only. Without measurable electrical fields and ions present at substrate level, no plasma induced charging damage of critical structures occurs. Microwave assisted plasma is highly environmentally compliant due to nearly complete dissociation of process gases like CF4.
Applying microwave (MW) plasma to remove thick resist such as used for micro-machinery and microelectromechanical systems (MEMS) requires adequate endpoint detection to limit the unnecessary exposure of sensitive structures and surfaces to reactive gases. The unique properties of microwave assisted plasma allow easy and accurate endpoint detection using temperature sensors.
RF-plasma exposes the substrate to a relative high amount of charged species (ions) related to the energy distribution in the plasma. These ions transfer kinetic energy when hitting the surface of the substrate causing it to heat up. Depending on the process conditions, this heating-up is critical for the integrity of the substrate, hence cooling is required to stay within the specified temperature range. In the case of Muegge’s microwave assisted remote plasma source, the energy transfer is extremely low, no charged particles are released into the process chamber. As a result the substrate remains cold, the only heat-source it is the chemical reaction of the exothermic resist removal. The related temperature change can be used to track the progress of the cleaning process.
Although optical endpoint detection is mature and reliable, it requires substantial invest into additional equipment like interferometry and sophisticated control algorithms. For less demanding applications (e.g. photoresist removal) customers are looking into less complex solutions. Temperature as indicator for the reaction progress is an obvious choice but usually not easy to track because of physical side-effects of the plasma generated species (e.g. temperature raise due to sputtering).
Radical based photoresist removal allows precise tracking of
the heat released by the chemical reaction only. Data collection is easy to realize, reliable and repeatable.
Different applications require different technologies for endpoint control in high volume production. Using the measurement of substrate`s temperature during photoresist-removal offers a reliable method to detect endpoint, however, RF-plasma technology produces too much noise to separate the reaction-heat from other heating-up mechanisms. Microwave-assisted photoresist removal allows to collect temperature data with sufficient resolution for endpoint detection. Temperature sensors work reliably for the typical pressure range of microwave-assisted plasma, no additional hardware is required. The signal is stable and free of interference as it only depends on the chemical reaction taking place on the substrate. Over-etch can be easily derived from the endpoint, changes in the process chamber (e.g. leaks, gas-flow) will reflect in the endpoint curve and help to adjust preventive maintenance cycles. The detection is inherently very stable and does not need re-calibration in case of process adjustments. It is a feature of the Muegge microwave systems (STP series) and proven in high volume production.
Microwave plasma is the perfect solution for removing thick resist such as used for 2- and 3-dimensional structures in micro-machinery and microelectromechanical systems (MEMS). The advantage of microwave (MW) plasma over RF-plasma is crucial - organic materials can be cleared without damage or altering of subjacent layers and structures.
Whereas RF-plasma is the most common method to etch thin layers, it is not ideal to remove thick photoresist from 2- and 3-dimensional structures because of its anisotropic (directional) nature. The reactive species generated in the RF-plasma are directed towards the substrate and not able to remove the resist within 3D-structures, i.e. if protected from direct exposure. Additionally, heat transfer due to sputtering will cause thick photoresist (PR) to polymerize during anisotropic etching when getting hot.
Manufacturing of MEMS requires a wide range of PR-removal applications. Microwave-plasma allows to adjust for very high striprate at high selectivity but also the etching on top of CF4- and O2-sensitive materials. MW assisted plasma is specially designed for fast etching without attacking sensitive parts of the substrate. It provides the capability for complete resist removal at high speed (>200um/h) and high selectivity with no altering of underneath layers. The radicals generated only produce chemical reactions at the surface, leading to pure chemical etching at high rates with extremely low thermal load, thus keeping the effect on the substrate as low as possible. No measurable electrical fields and ions are present at substrate level, hence no plasma induced charging damage of critical structures occurs.
MW assisted plasma is highly environmentally compliant due to nearly complete dissociation of process gases like CF4.