Introduction
With the rapid development of technology, there is a need for lighter, more efficient, smaller, multifunctional and high quality laser equipment for electronics, medical therapy, biology and materials. Currently common lasers are available in infrared and visible wavelengths. Traditional laser tools, processes and technologies suffer from low efficiency, complex operation, high costs, restricted range, severe losses and low accuracy. UV lasers have been repeatedly researched by scientists in recent decades for their relatively high coherence, convenience, stability and reliability, low cost, tunability, small size, high efficiency, accuracy and practicality.
2. UV lasers
UV lasers are mainly divided into gas UV lasers and solid UV solid state lasers. The working medium reaches an excited state by absorbing external energy under the action of the pump source, and after the particle number inversion gain is greater than the loss, the light is amplified and part of the amplified light is fed back to continue the excitation thus generating oscillation in the resonant cavity to produce the laser. Gas media are mainly used in pulsed or electron beam discharges, where the collisions between electrons excite the gas particles from low energy levels to high energy levels to produce excited jumps to obtain UV lasers. The solid medium is a non-linear frequency doubling crystal that produces outwardly radiating UV laser light after one or more frequency transitions. Excimer and all-solid-state UV lasers are commonly used for laser processing and handling.
2.1. Excimer lasers
The main gas UV lasers are excimer lasers, argon ion lasers, nitrogen molecular lasers, fluorine molecular lasers, helium cadmium lasers, etc. Excimer lasers etc. are commonly used for laser processing. Excimer lasers are gas lasers with excimer as the working substance. They are also pulsed lasers and have been of great research interest since the first excimer laser was created in 1971. Excimer is an unstable compound molecule that breaks down into atoms under certain circumstances. Repetition frequency and average power are the basis for judging excimer lasers. A certain proportion of rare gases such as Ar, Kr and Xe mixed with halogen elements such as F, Cl and Br are the main working substances of UV gas lasers, which are pumped by means of electron beams or pulsed discharges. When atoms of noble and rare gases in the ground state are excited, the electrons outside the nucleus are thus excited to higher orbitals so that the outermost electron layer is filled and combined with other atoms to form quasi-molecules, which then leap back to the ground state and break up into the original atoms. Liquid xenon was the working substance for the early excimer lasers. Today's excimer lasers also include the ArF laser at 193 nm, the KrF laser at 248 nm and the XeCl laser at 308 nm.
2.2. Solid-state UV lasers
The outstanding advantages of all-solid-state UV lasers are their convenient small size, high reliability and operational stability. The most commonly used is the usual Nd:YAG crystal for LD pumping, which is then frequency doubled.
The main steps in the generation of a UV solid-state laser are firstly the pumping of the light source in the laser onto the intensifier medium to achieve particle number inversion, the formation and oscillation of the fundamental red light in the resonant cavity, then the doubling of the frequency in the cavity by one or more non-linear crystals, and finally the output of the desired UV laser from the resonant cavity after transmission and reflection. UV solid-state lasers are usually obtained by using LD diode pumping and lamp pumping methods. All-solid-state UV lasers are LD-pumped UV solid-state lasers.
Nd:YAG (neodymium doped yttrium aluminium garnet) and Nd:YVO4 (neodymium doped yttrium vanadate) are two of the more common types of reinforced media crystals. A common method of enhancing resonant cavities is to use a small semiconductor laser diode LD pumped with a Nd:YVO4 laser crystal at a wavelength of 808 nm to produce near infrared light at 1064 nm. Compared to Nd:YAG, the Nd:YVO4 laser crystal has a larger gain cross section, four times that of Nd:YAG, a larger absorption coefficient, five times that of Nd:YAG, and a lower laser threshold. Compared to Nd:YAG, the Nd:YVO4 laser crystal has a larger gain cross section, four times that of Nd:YAG, a larger absorption coefficient, five times that of Nd:YAG, and a lower laser threshold. Nd:YAG crystals have high mechanical strength, high light transmission, long fluorescence life and do not require a harsh heat dissipation and cooling system.
3. Applications of UV lasers
UV laser processing has many advantages and is currently the technology of choice in the development of technological information. Firstly, the UV laser can output ultra-short wavelengths of laser light, which can precisely deal with ultra-small and fine materials; secondly, the "cold treatment" of the UV laser does not destroy the material itself as a whole, but only treats its surface; furthermore, there is basically no effect of thermal damage. Some materials do not absorb visible and infra-red lasers effectively, making them impossible to process. The biggest advantage of UV is that basically all materials absorb UV light more widely. UV lasers, especially solid-state UV lasers, are compact and small, simple to maintain and easy to produce in large quantities. UV lasers are used in a wide range of applications in the processing of medical biomaterials, forensics in criminal cases, integrated circuit boards, the semiconductor industry, micro-optical components, surgery, communications and radar, and laser processing and cutting.
3.1. Modification of surface properties of biological materials
In some treatments, many medical materials need to be compatible with human tissue or even repaired, such as ultraviolet laser treatment of intraocular diseases and experiments on rabbit corneas which sometimes require changes in biological protein properties and biomolecular structures. After adjusting the optimal pulse parameters of the excimer UV laser, the experimentalists then irradiated the surface of medical biomaterials with 100 nm, 120 nm and 200 nm lasers respectively, thus improving the physicochemical structure of the material surface and not changing the overall chemical structure of the material, and making the treated organic biomaterials significantly more compatible and hydrophilic with human tissues through comparative experiments with cultured biological cells, which is of great help in medical biological applications.
3.2. In the field of criminal investigation
In the field of criminal investigation, fingerprints have been used as important biological evidence left at the crime scene by suspects in criminal cases since it was discovered that fingerprints are as unique as DNA. Once old methods can lead to sample damage and make it difficult to collect and store exhibits. The current research has outstanding results for non-penetrating object surface fingerprints, such as tape, photographs, glass, etc. appearances. UV luminescence imaging" and "UV laser reflectance imaging" are used to observe and record the detection and collection of fingerprints by UV laser irradiation of potential fingerprints through band-pass filters at 266 nm and 340 nm respectively. Seventy per cent of the 120 samples tested in the experiment were successfully detected. The UV short-wave technique increases the success rate of potential fingerprints, and the ease and speed with which the optical properties can be controlled makes it promising for use in courtroom science. On-site saliva spots, exfoliated cells, bloodstains, hair with hair follicles and other common biological specimens can be detected with UV detection. However, when the short-wave 266 nm UV laser was used to irradiate biological samples at a fixed distance and at different durations and then to extract DNA, it was found that the short-wave 266 nm UV laser had a serious effect on the DNA results of five common types of biological evidence: fingerprints, bloodstains, saliva spots, shed cells and hair with hair follicles, but only to a lesser extent on the detection of biological DAN for hair including hair follicles, saliva and blood spots. Short-wave UV lasers can affect some DNA biomaterials, so the extraction method should be carefully chosen for its evidentiary value during forensic investigations.
3.3. UV laser applications on integrated circuit boards
The production of a wide range of circuit boards in industry, from the initial wiring to the production of tiny precision embedded chips requiring advanced processes, flexible circuits within integrated circuit boards, laminated circuits in polymers and copper all require micro-hole drilling and cutting, as well as the repair and inspection of materials on the boards, often requiring the use of micro-fabrication and processing. Laser micromachining technology is clearly the best choice for the processing of circuit boards. The laser does not come into contact with the product to be processed during the process, effectively avoiding mechanical forces, resulting in rapid processing, high flexibility and no special requirements for the workplace, which can reach sub-micron magnitudes through the precise setting of laser parameters and research design. The more traditional drilling methods used on circuit boards are the use of UV lasers and CO2 lasers for non-metallic marking (CO2 lasers with a wavelength of 10.6 μm are used for marking non-metallic materials; wavelengths of 1064 nm or 532 nm are generally used for marking metallic materials). At present, UV laser processing technology is still mainly used, which can achieve micron-level processing, high accuracy, can produce ultra-fine micro-zero devices, can be applied to less than 1 μm spot of the laser beam of micro-hole processing. However, CO2 lasers are mainly used for holes between 75 and 150 mm and are prone to misalignment in small holes, whereas UV lasers can be used for holes up to 25 mm with high accuracy and no misalignment. For example, in the "cold" processing of copper-clad circuit boards with UV femtosecond lasers, a comprehensive balancing method is used to obtain the optimum process parameters, and the selective etching properties are then used to achieve high-quality, high-efficiency micro line etching of copper-clad surfaces with a line width of 50 μm and a line pitch of 20 μm.
3.4 Processing and preparation of micro-optical components
In the age of information technology and the rapid development of modern industry, the need to build more experimental systems in a smaller space and to achieve more functions requires the accelerated development of information technology and, more importantly, the production of smaller, miniaturised and fully functional devices that only process the chemical bonds on the surface of the material. It has important applications and research value in the fields of military radar communication, medical therapy, aerospace and biochemistry. More in-depth cutting and optimisation and research and development of applications on micro-optical components at the nanoscale are possible, transforming the functions and properties of traditional optical components. Micro-optics have the advantage of being easy to mass produce, easy to array, small, light and flexible, but the main material is quartz glass. Quartz glass is prone to cracking and cratering during application and handling and is a hard and brittle material, which significantly reduces its optical properties. As a result, the UV laser's direct writing "cold" processing technology has greatly improved the efficiency of micro-optical devices, enabling the rapid processing of micro-optical components with high precision and fine structure without damaging the material, and allowing flexible processing of large and small batches with different requirements. While foreign research institutes have studied UV-UV processing of silicon wafers earlier, domestic research on silicon wafer cutting technology and facets was conducted only after a relatively late start. Optimised cutting of three silicon wafers of the same material (0.18 mm, 0.38 mm and 0.6 mm) with a minimum aperture of 45 μm and a machining accuracy of 20 μm, showing no cracks in the material, less thermal influence of the laser and less spatter.
3.4. UV laser applications in the semiconductor industry
Micromachining of semiconductor materials with UV lasers has received increasing attention in recent years. Thousands of dense circuit components are very common in integrated circuits, so some high-precision handling and processing methods are required, as well as some high-precision instruments and devices such as silicon and sapphire semiconductor materials and other semiconductor thin films of precision microprocessing by UV laser and study the spectral properties of the film, while the UV laser can also increase the utilization of light energy of silicon materials, but also make the silicon surface microstructure changes, which is conducive to the development of solar panels, such as two-dimensional micro-grating, etc.
4. concluding remarks
Through decades of development and research, the technology and applications of UV lasers have become more and more widespread and mature, and its most characteristic fine "cold" processing technology micro-processes and treats surfaces without changing the physical properties of the object, and is widely used in various industries and fields such as communications, optics, military, criminal investigation and medical treatment. The 5G era, for example, is generating demand for FPC processing. With the further development of the 5G industry and the pursuit of flexible OLED displays by major electronics manufacturers, the demand for FPC flexible circuit boards is growing rapidly, and with it, the demand for UV lasers. This trend will hopefully lead to a rapid development of UV technology itself to achieve greater breakthroughs in power and pulse width, as well as to new areas of application. The application of UV laser machines has made precision cold processing of materials such as FPC possible, while the gradual increase in FPC has driven the deployment of 5G, whose low latency characteristics provide unlimited opportunities for new waves of technological development such as cloud technology, the Internet of Things, driverlessness and VR. This is of course a complementary concept, and new technologies and applications will eventually drive further development of UV lasers.
As more and more new frequency doubling crystals and gain media emerge, the shorter the wavelength the higher the power of the UV laser will be used in the future in more industries to promote the development of all walks of life, UV lasers in the processing field more intelligent, efficient and accurate, high repetition rate, high stability is the trend of future development.
