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 uses photographic and chemical techniques to shape metal workpieces. After having the image of a design developed on the workpiece, a strong chemical solution is poured over the workpiece that selectively corrodes and removes excess material from unprotected areas leaving a flawless and sharp image or part. The areas to be saved or removed are selected using photographic techniques, such as photoresist imaging.
Companies use photochemical etching as an economical alternative to laser cutting, water jet cutting, and punching and stamping. The process allows for simple changes and adjustments during mass production. It produces parts with exceptionally high dimensional tolerances without burrs, sharp edges, or the need for product finishing. Once a design has been accepted and received, it can be completed in less than an hour.
The main advantage of this type of machining is its ability to produce precision parts impossible or impractical to be manufactured by traditional machining techniques. Photochemical etching is widely used in all industries such as aerospace, medical, life sciences, automotive, as well as in the field of electronics for producing printed circuit boards, silicon integrated circuits, pressure membranes and other small electronic components.
Parts produced are flat and thin; the smallest are usually in the order of ten microns. For more sophisticated methods of production, it is possible to produce products in the nano-scale, such as computer processors. Also, there is no limit to the complexity of the shape as long as it can be printed on the material. This added complexity adds negligible incremental costs to production.
Photochemical etching is a precision process for metal cutting and etching using specially formulated chemicals to create designs on flat sheets of metal. The nature of the process makes it possible to etch highly complex and intricate features into a wide variety of metals. The electronic process is used to produce electronic parts such as computer processors that have great complexity and require extraordinary attention to detail. Included in the process can be sophisticated coatings, optics, plasma generators, vacuum chambers.
The photo-tool is the photographic negative image of the desired profile or "artwork". The pattern is drawn using some form of engineering design software, such as drawing exchange format (DXF), illustrator, Computer Aided Design (CAD), or some other form of design software. The rendering of the design, including its parameters, is converted into a photo tool and printed to photographic film (either a silver halide or a diazo film) by a photoplotter or a laser-imaging system. Compensation factors are also added to the etching process. This is done by adding width to the profile or making the outside edges larger, while the inner edges (e.g. holes, slots, and notches) smaller. The following are some of the factors that affect photo-tool dimensions.
During plotting of the artwork into the photo-tool, dimensional variations can occur due to changes in temperature and humidity. This can be controlled by using thicker polyester films or glass as a photo-tool, or by doing the plotting in a controlled environment. If temperature and humidity variations cannot be eliminated, it is best to use "maskless" exposure techniques such as laser direct writing.
Since most etchants act isotropically (wet and plasma etching), undercuts are produced. Isotropic reaction is more prevalent on long etching processes; deep etches require longer etchant exposure periods. Thus, deeper etches result in more significant undercuts. The etch factor is expressed as the ratio of the undercut to the depth of etching. This must be considered while creating the photo-tool. The edges of the image plotted to the photo-tool must be adjusted to compensate for the undercut.
After adding the compensation factors, the final master image is then repeatedly plotted onto the film to maximize the output.
The workpiece is then cut and cleaned prior to application of the photoresist coating. This is to ensure that any oil, dirt, rust, or grease accumulated on the metal surface from its primary processing and handling will not prevent the photoresist from adhering. There are two methods of cleaning: chemical and mechanical. Chemical cleaning involves a mild pickling process where the material is suspended into a degreasing solution consisting of mild acids and degreasing agents. Mechanical cleaning, on the other hand, subjects the material to some form of scrubbing and application of mild degreasing solution. Chemical cleaning methods are preferred over mechanical due to the lesser damage applied onto the material.
A hexamethyldisilazane (HMDS) coating may also be applied to increase the adhesion of the photoresist. This ensures that the surface is hydrophobic, leaving a non-polar surface.
Photoresists are light sensitive, organic materials deposited on the surface of the workpiece. Photoresists can withstand the etching solution, leaving behind a defined image. When this coating is exposed to UV light, it becomes either soluble or insoluble to the developer solvent depending on the type used. The photoresist regions are either protected or exposed by the photo-tool. Photoresists are classified according to the type of image produced, chemical structure, and form. Photoresists according to the type of image produced are:
Positive photoresists are the type where the exposed areas become soluble to the developer solvent.
This type is opposite to the effect of positive photoresists wherein the exposed areas to UV light polymerize or cure becoming chemically resistant to the developer solvent.
Positive photoresists typically exhibit better image resolution in contrast with negative photoresists. However, they need longer exposure time, are more difficult to develop and remove, and are much more expensive.
The following are photoresists according to chemical structure.
In this type of photoresist material, free radicals are generated when exposed to UV light. These free radicals induce cross-linking reactions generating a cured film.
This process utilizes the photodecomposition of a photoactive material, usually diazonaphthaquinone (DNQ). This produces hydrophilic compounds which then reacts with water to form indene carboxylic acid, rendering the exposed part soluble.
In this type, acid is generated upon exposure to UV light which induce crosslinking reactions to form insoluble networks. This process is utilized for negative photoresists.
Photoresists can also be classified further according to form.
These are rolls of photoactive materials sandwiched by a separation sheet (top layer) composed of polyethylene film and a support or protective film (bottom layer) composed of polyester. The chemical structure of the photoactive material can be either photopolymeric or photo decomposing.
Liquid photoresist is applied by a variety of methods such as dip coating, spray coating, spin coating, etc. Among these, dip coating is the most popular due to its ease and low cost.
After application of the photoresist material, the photoresist is baked or heated to vaporize and release residual solvents. This is done usually at temperatures ranging from 90°C to 110°C. This process must be controlled since high evaporation rate can cause bubbles to form creating voids within the photoresist. Low evaporation rate, on the other hand, can cause a film to form on the surface which inhibits evaporation of the residual solvents.
For multilayered patterns, it is important to ensure proper alignment of the photo-tools. This is done by matching markers from the photo-tool and the features on the workpiece.
This process is where the image on the photo-tool or mask will be transferred to the workpiece with a photoresist. Exposure is usually done via ultraviolet waves where the wavelength is less than 400nm. For DNQ, the required wavelength is around 300nm to 450nm. Note that certain photoresist polymers only react at a certain wavelength. For producing smaller structures, it is desirable to use photoresist materials and exposure techniques that operate at the smallest wavelengths.
Mentioned below are several techniques employed for exposing the photoresist.
In this setup, the photo-tool is in direct contact with the photoresist material. Thus, the pattern is transferred at a scale of 1:1. This process requires light intensity to be uniform across the entire pattern. Slight diffraction can be seen at the edges of the structures. This process is fast and simple since the whole wafer or workpiece is exposed. However, this method is not suited to produce very small patterns. Also, because of the direct contact between the mask and the photoresist, the mask is prone to damage and contamination.
This is similar to contact exposure in which the imaging scale is 1:1. This is mostly done on textured substrates that require a gap between the mask and photoresist. The gap solves damage and contamination issues attributed to the contact exposure method. However, it does not offer better resolution than contact exposure.
This method involves projecting the mask (known as reticles, for this process) through reduction optics. In contrast with contact and proximity exposure, this method uses a stepper that projects one or a few patterns onto the wafer at a time. The advantage of this process is that since the reticle is a few multiples larger than the projected image, any defects on the reticle are reduced thereby improving the resolution.
This method does not need any photo-tool or masks to create an image on the photoresist. This utilizes lasers focused to a narrow beam that directly exposes regions on the photoresist pixel-by-pixel. Laser direct writing has the advantage of etching in the scale of tenths of a micrometer that is difficult for conventional exposure processes. However, depending on the resolution, imaging time takes several hours to complete.
This method involves scanning a focused beam of electrons onto the substrate enclosed in a vacuum. Like the laser direct writing, the electron beam directly exposes regions on the substrate to change their solubility. However, the type of material coating the substrate is not the usual photoresist. Electron beam uses electron beam (e-beam) resists which respond to electrons or deep UV light (UV waves with short wavelengths). This method can write in the scale of a nanometer mostly used in the manufacture of computer processing units.
A common problem after exposure is the development of standing waves. These are thin film interference effects caused by the partial absorption of light or UV waves by the layers of the photoresist material. PEB can be an optional or critical procedure which is achieved by subjecting the wafer to temperatures around 110-120°C, depending on the type of photoresist used. This is commonly done on chemically amplified photoresists seen in photocrosslinkers. PEB completes the photoreaction initiated by the UV light exposure by thermally catalyzing the chemical reactions.
The developer removes the photoresist during the developing process. The workpiece is submerged in a developer, an alkaline solution for positive photoresists, with an organic solvent used for negative ones. This process takes advantage of the different photoresist dissolution properties between the exposed and unexposed areas. For positive photoresists, the exposed areas are soluble, which will be dissolved by the developer. The opposite is true for negative photoresists. The development process is time and temperature sensitive. Overdevelopment may result in swelling of the photoresist, which results in image distortion.
After development, the resulting wafer is "hard" baked. This is usually done by rinsing it with distilled water and nitrogen blow drying. Next, the wafer is then subjected to high temperatures usually equal to the PEB, or around 120°C. This process ensures the wafers‘ thermal, chemical, and physical stability which enables them to withstand the etching process. In addition, solvent, water, and other residues are removed resulting in better adhesion between the photoresist and the wafer.
The etching process involves removing the unprotected regions of the wafer or substrate by means of chemical agents. This leaves behind the desired shape of the product. The etching process can be divided into two main types: liquid or wet etching, and plasma or dry etching.
Wet etching uses chemicals to remove the unprotected substrate, and starts with the oxidation of the substrate for removal. The types of chemicals used for this process vary depending on the type of metal with nitric acid used for magnesium plates and ferric or hydrochloric acid used for copper plates. Once the substrate is removed, the workpiece passes through the diffusion and convection process where the dissolved oxidized substrate is removed and transported away.
Since liquid molecules can freely move in any direction, the chemical reaction may proceed in all directions; thus, an isotropic reaction. This type of reaction can remove the substrate under the photoresist producing undercuts. An anisotropic reaction is then desired which is achieved by etching according to crystalline orientations of the substrate or by using special chemical mixtures. However, this is only applicable for certain materials. This limits the wet etching process from producing microstructures less than a micron.
This process utilizes high velocities of gaseous ions to remove the material physically by erosion; although, chemical reactions may still be utilized to aid in the removal. Collision of ions to the substrate removes material in one direction only. This eliminates the undercut problem seen from the wet etching process. The gases and removed material are then expelled out by a vacuum system. However, this process can still leave deposits on the lateral surface of the etched material. This is addressed by another gas that is reactive with the substrate. This gas reacts with the exposed substrate surfaces removing particle deposits. Exposure to this second gas is controlled since this produces an isotropic reaction.
Another form of dry etching is plasma etching. This process uses high speed plasma of either ions or inert atoms or radicals. This also erodes the substrate while chemically combining with the eroded material and the exposed surface. A thin film is created on the surface producing a layer that prevents any deposits or isotropic reactions.
Dry etching can also be done by bombardment of reactive ions. Again, collision of the high velocity gasses removes the unprotected substrate with the aid of the chemical reaction. To address particle deposits on the lateral walls, oxygen is introduced creating an oxide layer. This passivates the lateral walls, making it unreactive to the ions.
After the desired substrate structure is created, the photoresist material is then removed. This must be done as quickly as possible, without causing damage to the substrate. This can be done in different ways. One method is by using solvents such as acetone, NMP (1-methyl-2-pyrrolidone), DMSO (dimethyl sulfoxide), or by proprietary read to use stripper mixtures. Solvents break down the structure of the resist layer usually with the aid of a heated environment. Another method is by oxygen combustion. Photoresists with too high cross linking which cannot be easily removed by chemical removers are combusted with O2 plasma.
There are many machines available to perform photochemical etching. These machines are important in today's society as they enable precise and cost-effective manufacturing of intricate parts and components used in various industries, such as electronics, aerospace, automotive, and medical, contributing to technological advancements and economic growth. Below, we discuss many photochemical etching machines known for their capabilities available in the United States and Canada:
Features: The LPKF ProtoLaser U4 is a high-precision laser system designed for rapid PCB prototyping and precision photochemical etching. It offers excellent accuracy and resolution, making it suitable for intricate designs. This machine incorporates an intuitive software interface for easy operation and precise control.
Features: The TTI-MED DTS 400 is a versatile and reliable photochemical etching machine. It is known for its ability to handle thin metal foils with high precision. This machine comes equipped with advanced controls, ensuring consistency and repeatability in the etching process.
Features: The Posalux UC 1000 is a highly automated photochemical etching machine widely used in the electronics industry. It offers efficient processing of large batches of PCBs with intricate designs. The machine's advanced handling capabilities and precise tooling contribute to its popularity.
Features: The Technics PEII is a well-regarded photochemical etching machine known for its robust construction and reliability. It supports various substrate materials and can accommodate a wide range of design complexities. The user-friendly interface makes it a popular choice for both small-scale and large-scale production.
Features: Ortlinghaus is a renowned manufacturer of photochemical etching machines used in diverse industries. Their machines are known for their ability to process a wide range of materials, including metals, plastics, and composites. These machines are equipped with advanced features for precise control over the etching process.
It's important to note that the popularity of specific models may vary based on individual user requirements, industry trends, and technological advancements. Before making a purchase decision, it is recommended to conduct thorough research and consult with experts in the field to ensure the selected machine aligns with your specific needs. Additionally, for up-to-date information on the latest models and their popularity, it is best to reach out to manufacturers and industry experts in the United States and Canada.
Photochemical etching offers benefits such as low tooling cost, burr-free products, stress-free method of production, high precision, and micro (or even sub-micro) production capabilities. Also, additional features in the design such as lines, holes, slots, or complex geometries does not produce any incremental cost.
The aerospace industry relies on sophisticated detection systems using flat spring contacts and precision designed fuel cells. Most of these components are made from aluminum and titanium, which are lighter than steel, copper, or brass. Weight and space are critical aspects of aircraft and spacecraft construction. The precision tolerances possible with photochemical etching as well as its ability to use any type of metal make it the ideal process for producing aerospace components.
The fact that photochemical etching can produce high precision and customizable part designs, is the reason it is so widely used in the aerospace industry. Thicknesses below 0.0005" to 0.062" can easily be done through photochemical etching while being impractical for the conventional stamping process.
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.
Due to its microscale production capability, photochemical etching is preferred than blanking, piercing, or stamping.
Photochemical etching is also used for jewelry and decorative purposes due to the ease of creating complex designs. As long as the artwork can be printed into a photo-tool, it can be produced by this process.
This is the most popular application of photochemical etching. The mass production of circuit boards pushed the emergence of the photochemical etching process. There are certain advantages that make this process more viable compared to mechanical machining such as burr-free products, ability to create complex designs, and fast prototyping.
A PCB helps to connect active and passive electronic components with pads, tracks, and lines placed on a laminated copper sheet. The PCB etching process includes the removal of copper clad from the substrate surface using chemicals to create a flawless PCB.
These include all electronic components, sensors, semiconductors, electrical contacts, and so forth. A popular electronic component that is produced by photochemical etching are computer processors. Production at this scale is virtually impossible for mechanical machining processes.
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 requires that the blocking processes be precision engineered and meticulously produced. Photochemical etching is an ideal way to produce flat sheets of protectors that can shield devices from electrical or magnetic fields. It is an economical method for producing electronic component‘ enclosures and packaging materials that protect against electromagnetic and radio frequency interference.
In the medical field, there is a rapidly growing movement and emphasis on disease prevention, early diagnosis, treatment, and repair of life threatening issues. The wide array of devices created by photochemical etching include implants such as pacemakers, neurotransmitters, and orthopedic components. Since medical processes demand a quick response and extreme accuracy, photochemical etching is the first choice due to the fact that it can customize implants to precisely fit the needs of a patient.
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.
Aside from its low cost and highly efficient production methods, photochemical etching has the ability to produce a wide and endless range of precision prototypes, finished components, and high tolerance assembly parts quickly and efficiently for any manufacturing and industrial operation. All metals are capable of being chemically etched regardless of their hardness, softness, fragility, thickness, and size.
Photochemical etching is a clean and scalable way to make precision custom metal parts. It removes the hassles of other fabrication methods and is capable of in process adjustments to meet changing parameters.
The process of PCM does not require tooling, die processing, or other mechanical manipulations. The removal of casting, grinding, and shaping of metals to produce production equipment greatly reduces production time and production costs.
Other metal shaping processes bend and change the properties of metals through the manufacturing process, which places stress on the metals structure. With PCM, the internal structure of a workpiece remains unchanged after undergoing chemical etching. The hardness, grain structure, and ductility of a metal is unaffected and maintain its original state.
Parts and components that have repeating patterns and intricate designs can easily and efficiently be produced to meet the demands of the highest tolerances.
A common problem with any form of metalworking are unwanted bits, flakes, burrs, and flaws that have to be machined off a workpiece after it has been processed. In many cases, this requires intense metal working and several man hours that add to the cost of a part. With PCM, this particular process is unnecessary. Once a part or component is processed, it is ready for shipment without the need of secondary finishing. There are no stresses, no burrs, and no changes to the metal molecularly or structurally.
A major reason that so many manufacturers choose photochemical etching is the exceptional adherence to tolerances, which are ± 0.025 regardless of a part's complexity.
<Metal etching is capable of shaping over 2000 different types of metals in a very short amount of time with the highest accuracy and precision.
Acid etching, also known as chemical etching or photo etching, is the process of cutting a hard surface like metal by means of a specially formulated acid for the process of etching in order to allow for the creation of a design onto the metal...
Metal etching is a metal removal process that uses various methods to configure complex, intricate, and highly accurate components and shapes. Its flexibility allows for instantaneous changes during processing...
EMI shielding is a technique of creating a barrier that prevents leakage of strong electromagnetic fields that can interfere with sensitive devices and signals. They can be installed to isolate the electromagnetic field source or as an enclosure of the device that needs protection...
Laser cutting is a non-traditional machining method that uses an intensely focused, coherent stream of light called lasers to cut through the material. This is a type of subtractive machining process where the material is continuously removed during the cutting process...
Radiofrequency (RF) shielding is the practice of blocking radiofrequency electromagnetic signals that cause radio frequency interference (RFI). RFI can disrupt the electrical circuits of a device from working normally...
Water jet cutting is a manufacturing process that uses high pressure jets of water provided by pressurizing pumps that deliver a supersonic stream of water to cut and shape various types of materials. The water in water jet cutting is...