Excimer Technology: Understanding the science behind high-energy UV emission
The term excimer (a portmanteau of “excited” and “dimer” coined in the 1960s) refers to a temporary atomic state in which high-energy atoms form short-lived molecular pairs, or dimers, when electronically excited. These pairs are known as excited dimers. When these excited dimers return to their original state, the excess energy is released as ultraviolet (UV) light, specifically in the form of ultraviolet C (UVC) photons.
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What Makes an Excimer Different?
By definition, an excimer refers to a dimer (a molecule made of two atoms) where both atoms come from the same element or species. For example, in a xenon (Xe) excimer lamp, high-energy xenon atoms (Xe) combine to form excited Xe2 dimers. When these dimers return to their ground state, they release UV photons with a wavelength of 172 nm. This wavelength is particularly useful in industry for surface activation, such as cleaning or modifying materials at a molecular level.
On the other hand, when molecules from two different elements form an excited complex, it's called an exciplex. A common example is the krypton-chloride (KrCl) exciplex, which emits UV light at a wavelength of 222 nm. This particular wavelength is highly valued for its ability to effectively kill harmful microorganisms, making it ideal for disinfection applications.
Excimers and Excimer lamps: What's the Connection?
While the term excimer technically refers to homonuclear dimers (pairs of the same element), it is often used more broadly to refer to both excimer and exciplex molecules, especially when discussing their use in technology. This broader usage has led to the term excimer lamp, which refers to devices that generate excimer (or exciplex) radiation, typically through discharge-based methods.
How do excimer lamps work?
Excimer lamps are specialized emitters filled with noble gases and coated to irradiate specific vacuum-ultraviolet (VUV) wavelengths, depending on the gases inside the sealed quartz glass chamber.
When high-energy electrons are introduced into the lamp, a plasma discharge is created, also known as a dielectric barrier discharge. This plasma contains high-energy electrons that excite the noble gas atoms inside the lamp, triggering the formation of excited dimers.
As these dimers return to their ground state in nanoseconds, they release UV radiation at specific wavelengths. The emitted photons travel through the air, where they are partially absorbed by oxygen to form ozone on the substrate. This ozone then breaks the molecular bonds of the surface.
The result is the removal of some substrate molecules, leaving behind a chemically modified top layer. Ushio’s excimer lamps, which can emit UV-C light at wavelengths from 172 nm to 310 nm, offer this effect along the entire length of the lamp. The specific wavelength is determined by the specific noble gas mixture used in the lamp.
222 nm UV-C and its disinfection benefits
The 222 nm wavelength, also known as "far UV-C", is gaining attention for its ability to disinfect surfaces without damaging organic tissue. This means that 222 nm lamps and modules, unlike 254 nm UV-C lamps, can be used in areas where people are present, when used within the current exposure limits defined by national regulations. Ushio’s Care222™ UV disinfection solutions are based on this wavelength.
Industrial applications and the 172 nm wavelength
In industrial settings, the most commonly used excimer wavelength is 172 nm. With an energy of 7.23 eV, it effectively breaks the molecular bonds of most organic molecules—except for those between carbon and oxygen (C=O) and some inorganic oxides. This makes 172 nm ideal for surface modification like cleaning, activation, and matting.
- The substrate surface is covered by contaminants which deactivate the surface, these contaminants may be organic matter such as fats and/or oxides.
- An excimer lamp calibrated to emit 172 nm radiation can directly crack the bonds of molecules on the substrate surface, residues of which remain on the surface.
- In the air gap between the excimer module and the substrate, high-energy oxygen radicals and ozone are produced after being excited by the 172 nm radiation. The radicals oxidise the residues and therefore the residues are removed from the surface.
Increase Surface Energy and Wettability with Ushio VUV Light
Excimer irradiation ‘activates’ the surface by increasing the surface energy, which is measured in millinewtons per metre (mN/m). Surface energy determines the contact angle between a fluid and a solid surface. To ensure adhesion and homogenous distribution, in applications such as printing and gluing, a smaller contact angle must be achieved.
- Fluid surface energy > Substrate surface energy: Poor wettability and adhesion
- Fluid surface energy = Substrate surface energy: 90° contact angle, average wettability
- Fluid surface energy < Substrate surface energy: Good wettability and adhesion
Surface Activation Energy Requirements
- Solvent inks: ≥ 34 mN/m
- UV inks: 36 - 42 mN/m
- Water-based colour systems: 38 – 50 mN/m
- Coatings: 36 - 42 mN/m
While the surface irradiation of polymers and other composite materials may be the most popular usage for excimer, the treatment is also applicable to almost any surface. Cleaning, coating, and bonding can also be significantly improved when applying excimer technology to silicon wafers, metals, and glass. In some cases, total hydrophilisation can be achieved in which wetting of the entire surface occurs. At this point, the contact angle of the liquid reaches zero degrees, transforming the drop of liquid into a liquid film.
An alternative to Corona and Plasma treatment
As an alternative to corona- and plasma-based surface treatments, excimer’s cold light drastically reduces the excess heat produced by comparable IR methods.
In fact, the IR emissions from excimer are almost non-existent, as is the presence of any thermal stress to the substrate. This reduced thermal load means no damage to the surface structure, which in turn assures better bonding results.
Corona and plasma are both electrical discharge methods, so the object being treated receives a heavier impact on its surface. Electrical discharge occurs when fluid in the air surrounding a conductor is ionised, which in the case of corona and plasma, is created by applying a high-voltage between two electrodes.
Instead, Ushio excimer lamps utilise VUV light to treat the surface, so there is no discharge reaction or damage to your substrates, nor is a significant heat load applied to the surface. Ultimately, this lower heat load makes excimer an ideal solution for temperature sensitive substrates, such as foils.
The photons emitted from an excimer lamp maintain a relatively low energy compared to the accelerated particles in other treatment methods, which makes a noticeable difference on the surface itself. This ‘softer’ approach means the substrate surface does not become rough like it does with corona and plasma.
Viewing a discharge-treated surface under an electron microscope will reveal a moon like surface with large scattered holes, as a result of molecular bombardment. When viewing the same material type after excimer treatment, the surface remains smooth, and benefits from a higher level of homogeneity.
Thanks to the absence of an electrical field, excimer does not suffer from dust contamination like the aforementioned alternatives, and can be applied to all electronics, which would otherwise suffer from static related issues associated with a discharge.
These desirable properties are supplemented by some welcome technical specifications:
Introducing the Ushio ExciJet series
- Facilitation of line spectra and broad distribution
- Instant on / off functionality without heating or cooling cycles
- Compact size of ExciJet modules enables enhanced manoeuvrability
- Easy control system and built-in power supply
- Scalability allows for potentially unlimited lamps & modules
You may not realise it, but excimer is not a new technology. Ushio has been perfecting and discovering more applications for over a quarter of a century now, and the astonishing potential of excimer shows no sign of slowing. This has culminated in the release of its coveted ExciJet series which downsizes the technology to easily portable solutions:
- Compact, customisable plug-and-play modules for laboratory testing
- Robust, dimmable, mobile solutions for industrial applications
- High homogeneity and optimisation for the manufacture of semiconductors
- VUV output of over 170 mW/cm2 cleans up to 300 m/min in LED panel production
- Development & production satisfaction supplemented by an expert after-care team
Safe Far-UVC excimer technology is ready for human exposure
In general, ultraviolet light has the deserved reputation of being dangerous for humans, yet these wavelengths have beneficial uses. Mercury-vapour arc UV lamps are widely used in direct anti-microbial applications by emitting at 254 nm. At 254 nm, UV radiation gains enough intensity that it is able to penetrate the epidermal layer of skin and cause molecular changes in the layers beneath. These wavelengths are particularly damaging to the skin and eyes, leading in many cases to serious cases of mutation and illnesses such as cancer.
The excimer irradiation solutions from Ushio occupy customisable wavelengths between 170 nm and 310 nm, which are chosen for specific application suitability at each individual wavelength. Within the far-UVC region of 207 nm to 222 nm, it has been discovered that certain wavelengths are capable of disinfecting surfaces without causing the irreparable organic damage associated with other wavelengths.
222 nm has emerged as the safest for prolonged human exposure. At this range of intensity, the 222 nm UV radiation is unable to penetrate the top layer of skin or the eye-lens and simply deactivates any bacteria that may be present on the surface.
Our surface activation technology has been developed to concentrate 172 nm light on your chosen substrates. On the other hand, our PureRelease water purification component has proven 242 nm UV to be most effective in the instant disinfection of water at the point of dispensation (POD).
Experience VUV irradiation for yourself at Ushio’s Excimer Application Laboratory
Visitors to Ushio’s trade fair excimer demonstrations have been treated to exhibitions of surface activation, as various substrates were treated with an ExciJet 172 nm modular lamp, before revealing the result of an Ushio logo when displayed on the substrate when sprayed with water.
The live demonstration is typical of the applications tested at the Ushio Excimer Application Laboratory, and serves as an illustration of the simple set of processes Ushio follow to ensure optimal standards for your bespoke implementation.
Ushio’s excimer light technology allows the bonding of precision parts, such as optical & medical equipment, without adhesives or chemicals. By pressing two substrates together during treatment, a high strength and permanent bond takes place at a low pressure & temperature, with a faster annealing time.
The excimer technology developed by Ushio has already earned an excellent track record in the following industrial processes:
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Bespoke excimer lamp and housing systems can be designed on request, unlocking the immense application possibility for various materials and surfaces. With an in-built power supply and completely flexible specifications, the excimer lamp & module solutions can easily adjust to your needs. Not only can Ushio help you find the right product for your needs, we share years of research to help make recommendations for process and execution.
- Broad length options & dimmable power ranges: 55 mm 9 W lamp up to 2 kW with our 2,600 mm lamps
- Unlimited lamp capacity, each individually calibrated for different spectra examination
- Phosphor-generated wavelengths from 170 nm – 350 nm
Ushio’s excimer lamp and modular solutions are the result of 30 years invested in research & development, and still, new application advances are being made constantly. A recent motivation of the research team has been to enable printing of 3D electronics directly onto a pre-treated surface.
This advancement looks set to bring significant implications for products which are heavily reliant on weight reduction, such as those in the automotive and aviation industries. Of additional interest to these two important industries, excimer is able to facilitate the adhesive-free bonding together of certain materials.
It is even possible to bond certain polymers to quartz glass! The chart below illustrates examples of glass, mineral, and polymer types that can be bonded using excimer technology.
Ushio excimer irradiation solutions improve product quality
Overall, it really depends on what your process requires. At Ushio, we are more than capable of giving informed advice to our clients, which is why they come back to us every time.
The benefits of excimer are clear, surface activation is useful for all kinds of applications but often choosing the wrong method can lead to expensive complications later.
Excimer is unique in that it allows irradiation without allowing an electrical discharge to make direct contact with the substrate surface. This means that the treated surface will hold on to ink, adhesive, or lacquer for longer.
Ushio is able to ensure that its highly qualified development and production teams are at your disposal for an in-depth problem study, thorough research analysis, and extensive testing. You will receive immaculate implementation after your go-ahead, putting Ushio’s customer-driven focus to the forefront of your business.
Where quality prevails, Ushio excels. Which brings us to the meticulous after-service provided by the team to secure a 100% satisfaction guarantee, and ensures the design & solution are delivered to your exact requirements.
Are you still not convinced? We already have some huge references from Ushio clients and partners regarding excimer technology, and we are happy to share them with you. Click the icons below each testimonial to learn more about Ushio’s excimer successes.
“Sterilising medical instruments is of the utmost importance – every upgrade could save another life. To ensure consistent quality and do one better every time, we turn to reliable partners. Like Ushio. Our medical disinfection devices with sterilisation rely on excimer technology for accuracy. The Ushio team provided us with multiple design proposals, which helped us make an informed decision on the best lamp selection to continuously move forward.”
Mr. Lucas Meyer
Chief Technology Officer at SteriLux SA
“We could not have achieved these results without the Ushio excimer technology. It was the missing compact piece to our puzzle. The Ushio team has been dedicated to this project from start to finish, their expertise and service has only intensified the outstanding results the excimer technology generated for the project. Excimer fixed our surface adhesion to achieve outstanding flexibility and transparency the PhotonFlex project required. Ushio made it happen in the best possible time, size and way.”
M. Sc. Ludwig Pongratz
Project Manager for Micro and Nano Structuring at Fraunhofer-Institute for Laser Technology