This article takes and in depth look at about photochemical etching. You will learn more about topics such as:
Photochemical etching, also known as photochemical machining (PCM) or metal etching, is a non-traditional subtractive machining process that utilizes photographic and chemical techniques to shape metal workpieces. The process begins with developing a design image on the workpiece. A strong chemical solution is then applied, selectively corroding and removing material from unprotected areas to create a precise and sharp image or part. The areas to be preserved or removed are determined using photographic methods, such as photoresist imaging.
Photochemical etching is an economical alternative to laser cutting, water jet cutting, and punching and stamping. This process allows for easy modifications and adjustments during mass production. It produces parts with exceptionally high dimensional tolerances, free from burrs, sharp edges, and the need for additional finishing. Once a design is approved, production can be completed in under an hour.
The key advantage of photochemical etching is its ability to create precision parts that are either impossible or impractical to produce with traditional machining techniques. It is widely used across various industries, including aerospace, medical, life sciences, automotive, and electronics, for applications such as printed circuit boards, silicon integrated circuits, pressure membranes, and other small electronic components.
Parts produced through photochemical etching are typically flat and thin, with dimensions as small as ten microns. For advanced applications, production can extend to the nanoscale, including components like computer processors. The complexity of the shapes produced is virtually limitless, and adding intricate designs incurs only minimal additional costs.
Photochemical etching is a precision technique for cutting and etching metal using specially formulated chemicals to create detailed designs on flat metal sheets. This process enables the production of highly complex and intricate features across a variety of metals. It is commonly used in the electronics industry to manufacture components like computer processors, which demand exceptional detail and precision. Additionally, photochemical etching can be applied to produce sophisticated components such as coatings, optics, plasma generators, and vacuum chambers.
The photo-tool represents the photographic negative of the desired design or "artwork." The design is created using engineering software such as DXF, Illustrator, or Computer-Aided Design (CAD). This design is then converted into a photo tool and printed onto photographic film (either silver halide or diazo) using a photoplotter or laser-imaging system. Compensation factors are applied to adjust for the etching process, which involves slightly enlarging the outer edges of the profile and reducing the size of inner features like holes, slots, and notches. Several factors can affect the dimensions of the photo-tool.
Dimensional variations can occur during the plotting of the artwork onto the photo-tool due to fluctuations in temperature and humidity. To mitigate these issues, thicker polyester films or glass can be used as the photo-tool, or plotting can be performed in a controlled environment. If it is not possible to control temperature and humidity, employing "maskless" exposure techniques, such as laser direct writing, is recommended to achieve precise results.
Most etchants act isotropically (such as wet and plasma etching), which results in undercuts. Isotropic etching is more pronounced during long etching processes; deeper etches require extended exposure to the etchant, leading to more significant undercuts. The etch factor, which is the ratio of the undercut to the etching depth, must be considered when creating the photo-tool. Adjustments to the edges of the design on the photo-tool are necessary to compensate for these undercuts.
After accounting for these compensation factors, the final master image is repeatedly plotted onto the film to optimize the output and ensure consistent results.
Before applying the photoresist coating, the workpiece is cut and cleaned to remove any oil, dirt, rust, or grease from its surface. This ensures proper adhesion of the photoresist. There are two main cleaning methods: chemical and mechanical. Chemical cleaning involves a mild pickling process, where the material is immersed in a degreasing solution with mild acids and degreasing agents. Mechanical cleaning, in contrast, uses scrubbing and a mild degreasing solution. Chemical cleaning is generally preferred because it causes less damage to the material compared to mechanical methods.
To enhance the adhesion of the photoresist, a hexamethyldisilazane (HMDS) coating may be applied. This treatment creates a hydrophobic, non-polar surface that improves the bonding of the photoresist to the workpiece.
Photoresists are light-sensitive organic materials applied to the surface of the workpiece. They are designed to withstand the etching solution, creating a defined image. Depending on the type of photoresist used, exposure to UV light will either make the coating soluble or insoluble to the developer solvent. The photoresist areas are selectively protected or exposed based on the photo-tool. Photoresists are categorized by the type of image they produce, their chemical structure, and their form. The following are the categories of photoresists based on the type of image produced:
Positive photoresists are those in which the exposed areas become soluble in the developer solvent, allowing these regions to be removed while the unexposed areas remain intact.
In contrast to positive photoresists, negative photoresists undergo a chemical change when exposed to UV light, causing the exposed areas to polymerize and become chemically resistant to the developer solvent. As a result, the regions exposed to UV light remain on the workpiece, while the unexposed areas are removed.
Positive photoresists generally offer better image resolution compared to negative photoresists. However, they require longer exposure times, are more challenging to develop and remove, and are significantly more expensive.
The following are categories of photoresists based on their chemical structure:
Photopolymer photoresists generate free radicals when exposed to UV light, which initiate cross-linking reactions to create a cured film.
Photodecomposing photoresists use the photodecomposition of a photoactive material, such as diazonaphthaquinone (DNQ). This process produces hydrophilic compounds that react with water to form indene carboxylic acid, making the exposed areas soluble.
Photocrosslinking photoresists generate acids upon UV exposure that trigger cross-linking reactions, forming insoluble networks. This type of photoresist is used for negative applications.
Photoresists can also be further classified based on their form.
These photoresists come in rolls with a top layer of polyethylene film and a bottom layer of polyester film for protection. The photoactive material between these layers can be either photopolymeric or photodecomposing, depending on the chemical structure used.
Liquid photoresist can be applied using various methods, including dip coating, spray coating, and spin coating. Among these, dip coating is the most popular due to its simplicity and cost-effectiveness.
After applying the photoresist material, it is baked or heated to evaporate residual solvents. This process is typically conducted at temperatures ranging from 90°C to 110°C. Careful control of the baking process is essential, as a high evaporation rate can cause bubble formation, creating voids in the photoresist. Conversely, a low evaporation rate may result in a film forming on the surface, which can inhibit solvent evaporation.
For multilayered patterns, accurate alignment of the photo-tools is crucial. This is achieved by matching markers on the photo-tool with features on the workpiece to ensure proper registration.
During exposure, the image on the photo-tool or mask is transferred to the photoresist-coated workpiece using ultraviolet (UV) light. The exposure is typically performed with UV wavelengths shorter than 400 nm. For diazonaphthaquinone (DNQ) photoresists, wavelengths around 300 nm to 450 nm are required. It’s important to note that some photoresist polymers react only to specific wavelengths. For smaller structures, it is preferable to use photoresist materials and exposure techniques that operate at shorter wavelengths.
Several techniques are employed for exposing the photoresist, including:
In contact exposure, the photo-tool is placed directly in contact with the photoresist. This method transfers the pattern at a 1:1 scale. It requires uniform light intensity across the pattern and may result in slight diffraction at the edges of structures. While this method is fast and straightforward, it is not ideal for producing very small patterns. Additionally, direct contact between the mask and the photoresist can lead to mask damage and contamination.
Proximity exposure is similar to contact exposure but involves a gap between the mask and the photoresist. This gap helps mitigate damage and contamination issues associated with direct contact. However, it does not provide better resolution compared to contact exposure and is often used for textured substrates.
This method involves projecting the mask, known as a reticle, through reduction optics. Unlike contact and proximity exposure, this technique uses a stepper to project one or a few patterns onto the wafer at a time. The advantage of this process is that since the reticle is several times larger than the projected image, any defects on the reticle are minimized, resulting in improved resolution.
This method eliminates the need for photo-tools or masks by using lasers focused into a narrow beam to expose regions of the photoresist pixel-by-pixel. Laser direct writing offers the advantage of achieving extremely fine resolutions, down to tenths of a micrometer, which is challenging for conventional exposure techniques. However, depending on the resolution, the imaging process can take several hours to complete.
This method involves scanning a focused electron beam onto the substrate within a vacuum. Similar to laser direct writing, the electron beam directly exposes regions on the substrate, altering their solubility. Unlike traditional photoresists, this technique uses electron beam (e-beam) resists, which respond to electrons or deep ultraviolet (UV) light with short wavelengths. This method can achieve resolutions at the nanometer scale and is primarily used in the manufacturing of computer processing units.
A common issue after exposure is the formation of standing waves, which are thin film interference effects resulting from partial absorption of light or UV waves by the layers of the photoresist. To address this, Post-Exposure Bake (PEB) may be an optional or essential step. This process involves heating the wafer to temperatures around 110-120°C, depending on the type of photoresist used. PEB is commonly performed with chemically amplified photoresists, such as those used in photocrosslinkers. It completes the photoreaction initiated by UV light exposure by thermally catalyzing the chemical reactions in the photoresist.
The developing process involves removing the photoresist from the workpiece. The workpiece is submerged in a developer solution: an alkaline solution for positive photoresists and an organic solvent for negative ones. This process leverages the different dissolution properties of the photoresist in the exposed and unexposed areas. For positive photoresists, the exposed areas become soluble and are dissolved by the developer. Conversely, in negative photoresists, the exposed areas become insoluble. The development process is sensitive to both time and temperature, and overdevelopment can cause swelling of the photoresist, leading to image distortion.
After development, the wafer undergoes a "hard" bake process. This involves rinsing the wafer with distilled water and then blow-drying it with nitrogen. The wafer is then baked at high temperatures, typically around 120°C, similar to those used in the Post-Exposure Bake (PEB) process. Hard baking ensures the wafer's thermal, chemical, and physical stability, enabling it to endure the subsequent etching process. It also removes solvents, water, and other residues, improving the adhesion between the photoresist and the wafer.
The etching process removes the unprotected regions of the wafer or substrate using chemical agents, leaving behind the desired pattern. Etching can be classified into two main types: liquid or wet etching, and plasma or dry etching.
Wet etching involves using chemicals to remove unprotected areas of the substrate, beginning with the oxidation of the material to facilitate its removal. The chemicals used vary depending on the type of metal: nitric acid is commonly used for magnesium plates, while ferric or hydrochloric acid is used for copper plates. After oxidation, the workpiece goes through diffusion and convection processes, where the dissolved oxidized substrate is carried away and removed.
In wet etching, the chemical reaction can proceed isotropically because liquid molecules move freely in all directions, which may cause undercuts beneath the photoresist. To achieve an anisotropic reaction—where etching is directed according to the crystalline orientations of the substrate or through specialized chemical mixtures—is desired but applicable only to certain materials. This limitation restricts wet etching from producing microstructures smaller than a micron.
This process employs high-velocity gaseous ions to physically erode material from the substrate, though chemical reactions may also assist in the removal. The ion collisions remove material in a single direction, addressing the undercutting issue common with wet etching methods. A vacuum system expels the gases and removed material. However, this process can still result in deposits on the lateral surfaces of the etched material. To mitigate this, a secondary reactive gas is introduced. This gas interacts with the exposed substrate surfaces to remove particle deposits. The exposure to this second gas is carefully controlled to manage the isotropic reaction it produces.
Another form of dry etching is plasma etching. This method utilizes high-speed plasma composed of ions, inert atoms, or radicals. Plasma etching erodes the substrate while chemically reacting with both the eroded material and the exposed surface. This process creates a thin film on the surface, which helps prevent deposits and minimizes isotropic reactions.
Dry etching can also be performed using reactive ion bombardment. In this method, high-velocity gases remove the unprotected substrate with the assistance of chemical reactions. To prevent particle deposits on the lateral walls, oxygen is introduced to form an oxide layer. This oxide layer passivates the lateral walls, rendering them unreactive to the ions.
Once the desired substrate structure is achieved, the photoresist material must be removed promptly to avoid damaging the substrate. This can be accomplished using various methods. One approach involves solvents such as acetone, NMP (1-methyl-2-pyrrolidone), or DMSO (dimethyl sulfoxide), or specialized stripper mixtures. These solvents break down the photoresist layer, often aided by a heated environment. Alternatively, photoresists with excessive cross-linking, which are resistant to chemical removers, can be combusted using O₂ plasma.
Numerous machines are available for performing photochemical etching, playing a crucial role in modern manufacturing. These machines offer precise and cost-effective production of intricate parts and components across various industries, including electronics, aerospace, automotive, and medical fields. Their capabilities significantly contribute to technological advancements and economic growth. Below, we explore some of the leading photochemical etching machines available in the United States and Canada:
Features: The LPKF ProtoLaser U4 is a high-precision laser system designed for rapid PCB prototyping and precise photochemical etching. It delivers excellent accuracy and resolution, making it ideal for intricate designs. The machine also features an intuitive software interface, ensuring easy operation and precise control.
Features: The TTI-MED DTS 400 is a versatile and dependable photochemical etching machine, renowned for its capability to handle thin metal foils with exceptional precision. It is equipped with advanced controls, ensuring consistent and repeatable results throughout the etching process.
Features: The Posalux UC 1000 is an advanced, automated photochemical etching machine, favored in the electronics industry. It efficiently processes large volumes of PCBs with complex designs. The machine’s sophisticated handling features and precise tooling enhance its effectiveness and popularity.
Features: The Technics PEII is a highly respected photochemical etching machine, celebrated for its durable build and dependability. It handles a variety of substrate materials and supports diverse design complexities. With its intuitive user interface, it is a favored choice for both small and large-scale production runs.
Features: Ortlinghaus is a leading manufacturer of photochemical etching machines, widely utilized across various industries. Their machines are celebrated for their versatility in processing a broad spectrum of materials, including metals, plastics, and composites. Equipped with advanced features, these machines offer precise control over the etching process.
It’s important to remember that the popularity of specific models can vary depending on individual user needs, industry trends, and technological progress. Before making a purchase, it's advisable to perform comprehensive research and consult with industry experts to ensure the chosen machine meets your requirements. For the latest information on models and their popularity, contacting manufacturers and industry professionals in the United States and Canada is recommended.
Photochemical etching provides several advantages, including low tooling costs, burr-free results, and a stress-free production process. It offers high precision and can handle micro or even sub-micro scale production. Additionally, incorporating features like lines, holes, slots, or complex geometries into the design does not incur extra costs.
The aerospace industry depends on advanced detection systems that utilize flat spring contacts and precision-engineered fuel cells. These components are typically made from lightweight materials such as aluminum and titanium, which are preferred over steel, copper, or brass due to their reduced weight and space efficiency. Given the critical importance of weight and space in aircraft and spacecraft construction, photochemical etching is the ideal process for producing aerospace components. It offers high precision and can work with a wide range of metals.
Photochemical etching's ability to produce high-precision, customizable parts makes it a popular choice in the aerospace sector. It effectively handles thicknesses from below0.0005" to 0.062" which is challenging to achieve with traditional stamping methods.
Photochemical etching is preferred for this application due to its burr and residual stress-free products. Also, for custom gaskets, this process does not require high investment cost for fabricating hard tools.
Photochemical etching is preferred over blanking, piercing, or stamping due to its ability to achieve microscale production with high precision.
Photochemical etching is also employed in jewelry and decorative applications because it easily accommodates intricate designs. As long as the artwork can be transferred to a photo-tool, it can be effectively produced using this process.
The most popular application of photochemical etching is in the mass production of circuit boards. This process was driven by the need for efficient manufacturing methods in the electronics industry. Photochemical etching offers several advantages over mechanical machining, including burr-free results, the ability to create complex designs, and rapid prototyping.
A PCB (Printed Circuit Board) connects active and passive electronic components using pads, tracks, and lines on a laminated copper sheet. The PCB etching process involves chemically removing the copper cladding from the substrate surface to create a precise and flawless circuit board.
Photochemical etching is used for a wide range of electronic components, including sensors, semiconductors, electrical contacts, and more. A notable application of this process is the production of computer processors. Manufacturing at this scale is virtually unachievable with conventional mechanical machining methods.
There is an increasing need for radio frequency and electromagnetic shielding to keep electronic equipment working without interference. Electromagnetic interference (EMI) disrupts signals between electronic components and can block a cell phone signal and circuit boards. Radio frequency interference (RFI) interrupts and blocks radio signals. To block and prevent EMI and RFI disruptions, it is necessary to surround electronic components and cables with metal screens, sheets, and mesh.
The sensitivity of electronic instruments necessitates precision-engineered and meticulously produced shielding components. Photochemical etching is an ideal method for creating flat sheets of protectors that shield devices from electrical or magnetic fields. It offers a cost-effective solution for manufacturing enclosures and packaging materials that guard against electromagnetic and radio frequency interference.
In the medical field, there is an increasing emphasis on disease prevention, early diagnosis, and the treatment and repair of critical conditions. Photochemical etching is essential for creating a variety of medical devices, including implants like pacemakers, neurotransmitters, and orthopedic components. This method is preferred for its ability to deliver rapid and precise customization, making it ideal for producing implants that are tailored to the exact needs of patients.
The automotive industry is constantly adjusting and changing its designs to meet the demands of their highly competitive market. The flexibility and adaptability of photochemical etching is a perfect solution for the capacity growth and required precision of automobile production. A major contributing factor to the use of the photochemical process is its low overhead, which helps in keeping costs down for customers. Aluminum etched components are being used in electronic automobiles for efficient heat transfer for batteries. Photochemically etched stainless steel is commonly found in cars due to its corrosion resistance and heat and electrical conductivity.
Beyond its affordability and efficient production methods, photochemical etching excels at creating a diverse range of precision prototypes, finished components, and high-tolerance assembly parts swiftly and effectively for various manufacturing and industrial needs. It can process all metals, regardless of their hardness, softness, fragility, thickness, or size.
Photochemical etching is a clean and scalable technique for manufacturing precision custom metal parts. It eliminates many of the challenges associated with other fabrication methods and allows for adjustments during the process to adapt to evolving specifications.
Here are some of the primary advantages and benefits of using photochemical etching:
Photochemical etching achieves exceptionally fine detail and precision, capable of creating intricate and complex designs with tolerances as tight as a few microns. This makes it perfect for applications that demand high precision.
Unlike traditional machining methods, photochemical etching eliminates the need for costly tooling like dies or molds. This results in reduced initial costs, making it a cost-effective solution for small production runs and prototyping.
The etching process produces parts free from burrs, mechanical stresses, and heat-affected zones, which is crucial for maintaining the structural integrity of delicate or thin materials.
Photochemical etching works with a broad range of materials, including metals (such as stainless steel, aluminum, and copper), alloys, polymers, and even exotic materials like titanium, molybdenum, and nickel. This versatility makes it applicable across various industries and uses.
The process is highly scalable, accommodating both small and large production volumes efficiently. Whether producing prototypes, custom parts, or high-volume orders, photochemical etching can handle all with the same efficiency.
Due to its lack of tooling requirements and relatively short setup times, photochemical etching is ideal for rapid prototyping and quick design iterations. Design changes can be made swiftly and cost-effectively.
Photochemical etching is an environmentally friendly process that generates minimal waste. The chemicals used are often recyclable, making it a sustainable manufacturing option.
Parts produced through photochemical etching often exhibit enhanced corrosion resistance due to the smooth and uniform surfaces achieved during the process. This is especially beneficial for components exposed to harsh environments.
The photochemical etching process typically offers faster turnaround times compared to some traditional machining methods, which is advantageous for meeting tight production deadlines.
Photochemical etching excels in producing complex and customized designs, making it a preferred choice for industries like electronics, aerospace, medical devices, and automotive where unique or intricate components are required.
As a subtractive manufacturing method, photochemical etching minimizes material waste, offering both cost efficiency and environmental benefits.
This process ensures consistent quality and dimensional accuracy, with each part in a production run being virtually identical to the others.
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