This article gives you full knowledge about photochemical etching. You will learn:
- What is photochemical etching?
- The photochemical etching process
- Uses and benefits of photochemical etching
- And much more…
Chapter 1: What is Photochemical Etching and How Does it Work?
Photochemical etching, also known as photochemical machining or metal etching, is a non-traditional, subtractive machining process in which photographic and chemical techniques are used to shape the metal workpiece. This is done by exposing the workpiece to a strong chemical solution to selectively corrode areas or parts to be removed. Areas are selected using photographic techniques such as photoresist imaging.
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 tens of microns. For more sophisticated methods of production, it is possible to produce products in the nano-scale such as computer processors. Also, there is almost 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.
The downside of this process is the workpiece thickness limitations. The thickest can only be up to 2mm for most materials, while 6mm for copper. The thicker the part the larger the minimum through feature. The smallest through features that can be etched in the material is generally a minimum of 110% of the material thickness, meaning that if the material thickness is 1mm the smallest hole that can be etched is around 1.1mm diameter, based on science of the etching process. Also, at a mass production scale, waste and environmental effects can be an issue. Photochemical etching produces large chemical emissions. Acids, such as hydrofluoric (HF) acid, are highly toxic and can cause detrimental effects when not treated properly. Note most etching facilities have highly sophisticated emission filters and process controls to meet the high EPA standards.
Chapter 2: The Photochemical Etching Process
Photoetched production of electronic parts such as computer processors, a highly complex level of production is involved. This may include sophisticated coating devices, optics, plasma generators, vacuum chambers and so forth. Below are the major steps involved in the photochemical etching process.
The photo-tool is the photographic negative image of the desired profile or “artwork”. The pattern is usually drawn using a CAD software which is then converted to the appropriate file format. The image is then printed to a photographic film (either a silver halide or a diazo film) by a photoplotter or a laser-imaging system. Compensation factors are also added for 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.
Temperature and Humidity Variations
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 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 photoresists 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 or Wet Film
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 on 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 requires 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 is reduced thereby improving the resolution.
Laser Direct Imaging
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.
Post Exposure Bake (PEB)
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.
In the simplest terms, developing is done to remove parts of the photoresist by submerging it in a developer. The developer is usually an alkaline solution for positive photoresists, while an organic solvent is used for negative ones. To further elaborate, 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 then 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.
For the name itself, wet etching uses liquid chemicals to remove the unprotected substrate. The wet etching process starts by the oxidation of the substrate for removal. This is achieved by oxidizing components such as hydrogen peroxide or nitric acid. Next would be the dissolution of the oxidized substrate using substances such as hydrochloric acid, hydrofluoric acid, and phosphoric acid. The last step is the diffusion and convection process where the dissolved oxidized substrate is removed and transported away from the wafer, ensuring a homogenous solution in contact.
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 gases 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.
Photoresist Removal or Stripping
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.
Chapter 3: Photochemical Etching Applications
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.
Because of its high precision and customizable part designs, photochemical etching is 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.
Gaskets and Seals
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.
Fine Filters and Screens
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.
Printed Circuit Boards (PCB)
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.
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.
EMI and RFI Shielding
Photochemical etching is an economical method of producing electronic components’ enclosures and packaging materials that protects against electromagnetic and radio frequency interference.
- Photochemical etching or machining is a non-traditional method of machining that utilizes photographic and chemical techniques. This subtractive machining process is done by selectively corroding regions on the substrate, separated by a coating known as the photoresist.
- The major steps involved in photochemical etching are photo-tool and substrate preparation, photoresist coating, exposure, development, and photoresist stripping. Baking is also done usually after chemical and photoinitiated processes such as before and after exposure.
- Dimension compensation in photo-tool plotting are added to account for the effect of changing temperature and humidity.
- Photoresists are light sensitive materials that become either soluble or insoluble upon exposure to ultraviolet radiation. The difference in solubility across the photoresist coated substrate selectively exposes areas to be dissolved by the etchant.
- Exposure techniques are contact, proximity, projection, laser direct writing or electron beam. All of these achieve one goal—to change the solubility of the photoresist. Different exposure techniques are used depending on the type of resolution required and the dimensions of the structures to be etched.
- The etching process subjects the substrate to either solvent or high velocity gas streams. Solvents dissolve the substrate protected by the etchant, while high velocity gas, either inert ions or reactive ions, mechanically erode the substrate.
- Because of cost efficiency, high precision, and capability to produce micro scale products, photochemical etching becomes unparalleled from mechanical machining. Its main application is in the electronics industry.