Pre-cleaning of substrates before Physical Vapor Deposition (PVD) coating is an essential first step of surface preparation for thin film deposition. Removing contaminants such as oxides, hydrocarbons, and water molecules allows for good adhesion of deposition layers without defects.
One critical reason for pre-cleaning just prior to deposition is to remove the native organics (primarily oxidation) that form a barrier layer on the substrate surface from being exposed to regular atmosphere. Even as little as a couple nanometers of this natural barrier layer will prevent proper adhesion and can influence the conductivity of the layer stack, which is absolutely critical for semiconductors.
Many substrates coated with PVD were initially created by a machining or casting process that leaves lubricants from the molds. Substrates can also accumulate microscopic particles or factory-born residues during storage.
Weak bonding is usually the result of contaminants on the surfaces being coated. Strong adhesion is particularly important for high performance, high stress depositions like Diamond-Like Carbon (DLC) because they tend to fail peeling apart at surface boundaries.
Clean surfaces are critical for a good PVD coatings coatings that perform to high specifications. Impurities can dramatically affect the reliability of the electrical properties of semiconductors. For many applications, such as optical thin films like sunglasses or architectural glass, even relatively minor contaminants can create major problems in the reflective or transmission qualities of the finished product.
What is Etching?
System with RF Etch Video
Water Contact Angle (WCA) is a common method of measuring cleanliness by examining the degree to which water droplets ball or spread out on a surface due to the removal of contaminants. This method can be highly effective in detecting defects not even visible under high powered microscopes.
“Wettability” refers to the attraction of a liquid to a solid surface, allowing the fluid to flow and spread out when it is clean. A water droplet is placed on the cleaned surface and examined with an optical instrument called a tensiometer. This device measures the angle formed at the three-phase boundary where liquid, solid, and gas all meet. “Wettability” refers to the attraction of a liquid to a solid surface allowing the fluid to flow.
The tensiometer measures the degree to which the water droplets have steep walls balling up or low walls spreading out, which indicates how clean the surfaces are. The lower the wall angle, the cleaner the substrate is. This enables the creation of a more bondable adhesion due to the enhanced surface energy, without contamination of the substrate and resulting deposition.
Cleanliness of the deposition chamber is another key consideration. Because plasma etching with the right equipment can be performed in the deposition chamber, special care and regular maintenance must be taken to prevent the materials being etched from building up on the chamber walls and contaminating the deposition process.
What is Wet Etching?
Wet etching usually involves baths in solvents, or alkaline and aqueous solutions being applied to the surfaces. It is frequently combined with ultrasonic cleaning for the best results. Because wet etching involves liquids, it is usually done in a separate etching machine, then moved to the deposition chamber.
Wet etching is widely used with steel and high-performance metal components. The disposal of the chemicals used with wet cleaning have environmental impacts that must be given full consideration.
What is Dry or Plasma Etching?
Dry etching is performed with plasma rather than liquid etching fluids. The equipment set up is very similar to or can be the same as sputtering equipment.
Plasma etching works like sputtering, but in reverse. Rather than sputtering a material onto the substrate where the source material comes from the cathode (with the voltage potential) and is deposited onto the substrate (which is the anode in the circuit), the substrate becomes the cathode and the impurities that reside on its surface are sputtered off.
This can be done just to remove impurities. Another option is to go slightly past this layer to create the desired surface morphology in the underlying prepped surface, thereby promoting better adhesion of the deposited film that follows.
Prior to deposition, a bias is applied to the substrate in the presence of a low-pressure gas to create a plasma in the vicinity of the substrate surface. This plasma can be used in a variety of methods to clean, oxidize, or make coarser or smoother, the surface of the substrate for better adhesion and reliability of the deposition.
There are generally two types of dry etching: plasma etching conducted in the standard microwave range of 13.6MHz; and Hydrogen Gas plasma such as Ar + H2 which helps to lower operating voltage and better cover difficult shaped surfaces. Both are widely used in the electronics field.
Most commonly, RF is used to excite the gas and utilizes high speed pulses of plasma to etch the surface. Sometimes, this can be achieved using , albeit typically at a lower plasma density.
What are the Advantages of Plasma Etching?
Plasma etching is a preferred technique for many types of applications because:
- Plasma etching can improve the physical or chemical properties of substrates, including hardening them.
- With the proper equipment, plasma etching can be done in the sealed sputtering chamber without breaking the seal before deposition, reducing contamination.
- Plasma etching can make for better adhesive properties between etched materials.
- The breakdown from roughening the etched material is minimal compared to wet etching.
- Plasma etching can be used for patterned etching with masking.
- In many ways, plasma etching is more controllable and less risky than wet etching with acids and solvents.
- The environmental impact of plasma is very low compared to liquid solvents.
- Plasma etching can activate polymer surfaces such as plastics or resins.
- Plasma etching can reduce production costs by creating better bonds that make possible the use of comparatively inexpensive glues with polymers.
Types of Pre-Cleaning Etching Methods
Most PVD systems use one of two techniques. With cluster systems, a pre-clean/etch module is used before loading into the deposition chamber without breaking vacuum. In most batch systems, a pre-clean/etch feature is incorporated into the deposition chamber directly. Several technologies exist for pre-cleaning and etching, each with their own unique advantages and disadvantages.
RF Glow Plate has the advantages of using relatively low energy, therefore having the least chance of doing physical damage to a substrate when removing contaminants. A limitation with RF glow is that current voltage and process control cannot be independently controlled, except within a limited range.
Gridded Ion Sources, when compared to glow discharges, are higher energy and have the ability to accelerate ions with electromagnetic or electrostatic forces. Ion sources allow for higher impact plasma etching and the control of current and energy independently over a much wider field. Gridded ion sources are frequently used for oxide etches where an entire oxide layer needs to be removed.
The lifetime of the grids and electron sources, and the fact that substrates frequently require cooling, do present a limitation of gridded ion sources because they are complex and require a lot of maintenance for production equipment.
Gridless Ion Sources, having fewer parts and reduced maintenance replacing electrodes, have an advantage of being able to utilize a broad range of ion energies of varying intensities, making it an easily scalable technology. It can be used on temperature-depending substrates such as polymers and plastics, and offers relatively low investment and operating cost solutions for many thin film applications. A limitation of gridless ion sources is that they are not applicable for oxide etches.
What does “Clean” Mean and How is it Measured?
Water Contact Angle (WCA) is a common method of measuring cleanliness by examining the degree to which water droplets ball or spread out on a surface due to the removal of contaminants which increases the surface energy. This method can be highly effective detecting defects not even visible under high powered microscopes.
“Wettability” refers to the attraction of a liguid to a solid surface, allowing the fluid to flow and spread out when it is clean. A water droplet is placed on the cleaned surface and examined with an optical instrument called a tensiometer that measures the angle formed at the three-phase boundary where liquid, solid and gas all meet. “Wettability” refers to the attraction of a liquid to a solid surface allowing the fluid to flow.
The tensiometer measures the degree to which the water droplets have steep walls balling up or low walls spreading out which indicates how clean the surfaces are. The lower the wall angle, the cleaner the substrate is. This enables the creation of a more bondable adhesion due to the enhanced surface energy without contamination of the substrate and resulting deposition.
Cleanliness of the deposition chamber is another key consideration. Because plasma etching with the right equipment can be performed in the deposition chamber, special care and regular maintenance must be taken to prevent the materials being etched from building up on the chamber walls and contaminating the deposition process.
Choosing the Best Method for Your Application
Several considerations should come into play when determining your best solution. The type of deposition, on what type and how large of a surface, how sensitive the substrate surfaces are to damage from temperature, the depth of cleaning and etching required, and of course, your budget, is what determines the best method for you.
For example, whether you are etching oxides or need to remove an entire layer of oxide, can make a difference between a grid or gridless setup. Optical depositions can range in surface areas from sunglasses to architectural glass, but both require very exact specifications to work.
Many electronic components are relatively temperature delicate, requiring lower levels of energy provided by glow plates to prevent heat damage from higher energy ion sources. The depth of cleaning required can make the difference between the bombardment intensity and duration of time required for etching critical calculations. Higher energy processes are usually more complex and have higher costs.
Scalability can be a major factor for many purchases.
The bottom line is that budget considerations are always an important part of any cost analysis. In some cases, processes have a higher initial equipment investment, but result in lower production costs over time. The best method for your application is typically some trade-off between all of these factors. Of course, bottom line final costs are usually the final deal maker or deal breaker consideration.
Conclusion
Good PVD coatings depend upon clean surfaces. Product failures most often result from contaminants that negatively affect the adhesion between the surfaces to be coated.
Many of the biggest film adhesion problems can be resolved with pre-cleaning, heat (for water drive off), or both. Cleaning and adhesion testing of the cleaned surfaces for quality control is an essential and often underestimated component of successful PVD coating.
Semicore’s CAPOS all-in-one PVD Sputtering or Thermal Evaporation System
Semicore’s breakthrough CAPOS thin film deposition system is the PVD Industry’s first cost effective “Open-platform” design, that is ideal for both precision R&D and batch production.
Flexible enough that it can be used for either sputtering or evaporation coatings, this platform is also used for Semicore’s CAPOS-CT series, as a highly versatile “Cluster Tool” platform that can be configured with multiple Process Modules (PM) and cassette-to-cassette operation that speeds production cycles. Read more…
For more detailed information on these units’ specifications please download the PDF.
Matt Hughes is President of Semicore Equipment Inc, a leading worldwide supplier of sputtering equipment for the electronics, solar energy, optical, medical, military, automotive, and related high tech industries. Please let our helpful support staff answer any questions you have regarding Pre-Cleaning & Etching for PVD Coatings and how to implement the best techniques and equipment for your specific Sputtering Systems and Evaporation Systems needs by contacting us at sales@semicore.com or by calling 925-373-8201.
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