The process of depositing materials as thin films is essential in the manufacture of semiconductor devices and circuits; in optics; in data storage and displays; in sensors and aerospace components; in medical and many other high-tech applications.
Is the process sputtering, thermal or electron beam evaporation? In each case, film quality is dependent to a major extent on the materials used, and thus on the quality of the suppliers behind them.
For engineers, process teams and procurement personnel choosing thin film materials is not merely a matter of selecting an appropriate metal, alloy, oxide or ceramic from the shelf.
The decision affects film uniformity, adhesion, electrical performance, optical behavior, contamination control, production yield, and long-term process repeatability.
A reliable material strategy begins with the application, the deposition method, the required film properties, and the supplier’s ability to deliver consistent, high-purity materials over time.
What Are Thin Film Deposition Materials?
Thin film deposition materials are the source materials from which the extremely thin films on a substrate in high vacuum arise.
The films may be nanometers or microns thick, but may determine, among other things, how a device will conduct electricity, how it reflects light, how it resists wear, how it binds to the next film, how it performs in demanding operational states, and more.
“Physical vapor deposition” or more commonly PVD is used when the material is converted from a solid source into a vapor phase before being deposited on a substrate; sputtering and evaporation techniques both exemplify PVD.
In sputtering energetic ions strike a solid sputtering target and eject atoms from the target surface, and those atoms then traverse the vacuum chamber of the deposition machine and condense as a thin film on the substrate.
With evaporation the source material is heated until it vaporizes and condenses on the substrate surface, either using thermal resistance heating or perhaps an electron beam of energy acting to vaporize the source material before condensation is attempted on the substrate.
Because a film so deposited is a derivative in the strictest sense of its source material, all the particulars matter: purity, density, grain structure, geometry, bond quality, surface state, and packaging cleanliness all affect deposition stability and film state.
Start With the Application, Not the Material List
The engineer’s first consideration should be to specify what the thin film must do in the final product. A film for semiconductor metallization is completely different to one used for optical coating, magnetic storage, diffusion barriers, adhesion layers, decorative finishes, and wear-resistant surfaces.
Consider: a conductive metal layer may require extremely low impurity levels, and tight control of electrical resistivity. An optical coating may be more concerned with refractive index, transparency, and absorption (and film stress).
A barrier layer may require excellent adhesion, dense morphology, and compatibility with adjacent films. A research and development project may require flexibility in custom compositions, whereas a production process may require that repeatability, availability from inventory, and batch-to-batch consistency receive priority.
Specifying a deposition material without first defining the application can easily lead to loss of adhesion, nonuniform coatings, contamination of the coating surface, drift in process, low yield, or inconsistent operation of the device.
Converse discussions with the best supplier will specify the substrate, the deposition method and chamber configuration, target or evaporation source requirements, film specifications, and production requirements.
Match the Material to the Deposition Method
Sputtering targets and evaporation materials have different requirements, somewhat dependent on the deposition process employed.
Sputtering targets should be made to produce stable contact with the plasma, to make efficient use of the bulk material, and to produce smooth films.
Composition, density, microstructure, finish and geometry all have a bearing on deposition rate, arcing, particles and film properties. Bonding to a backing plate for thermal/mechanical performance inside the sputtering chamber may also be required.
Evaporation materials should be made for satisfactory controlled vaporization. Melting point and vapor pressure, compatibility with the crucible, method of evaporation and cleanliness of the material may all be important. For some materials, particularly relatively low melting point metals, evaporation is easy.
For others, an electron beam is required, and for high melting point materials the use of special crucible liners may be necessary. If the material reacts with the crucible, spits, decomposes or evaporates too unevenly, the film will be defective or unstable.
This is why the judgment of the supplier is important. A material which looks right on paper may behave very differently inside a real specimen deposition chamber! The most experienced suppliers will therefore help customer ensure that the material form, purity and preparation method suit the process environment.
Evaluate Purity and Contamination Control
Purity is one of the most crucial aspects of thin film deposition. Even very low levels of undesirable species can adversely influence the electrical behavior, optical performance, adhesion, corrosion resistance or reliability of devices.
For semiconductor and high-tech coating uses, engineers will often request very pure materials with elaborate controls of trace elements.
While it is easy to concentrate on how pure a material is, it is risky just to think in terms of a single number- should also assess the manufacturer’s controls on the manufacturing process, cleanliness, handling, packaging, and willingness to provide documentation.
A very pure supplier could still supply material with a problem if it has not been properly handled, cleaned and packaged so as to avoid the addition of particles or contaminating materials to surfaces.
Contamination of sputtering targets can arise from raw materials selected, machining of the bulk, by-products of the machining, surface oxides, bonding materials or packaging. Contamination in the evaporation materials can arise from particulate contamination, improper sizing, crucible liner incompatibility or materials that become entrained in the film deposited.
If performance of the films is critical, discuss the purity requirements, lot traceability, handling requirements, and inspection procedures prior to ordering.
Consider Target Geometry, Bonding, and Backing Plates
Target performance is a function of composition, but also of geometry and bonding. Depending on tool and process, targets may be planar, rotary, circular, rectangular or custom. Selecting the wrong geometry may result in lower utilization or non-uniformity, or problems in installation and cooling.
Bonding may be critical for many sputtering applications. A target bonded to a backing plate may provide advantages in heat transfer, mechanical stability and reliability of process. Poor bonding may lead to hot spots, cracking, delamination or unstable deposition.
In high-power sputtering applications thermal management becomes more critical still–inadequate heat transfer may result in shortened target life or undesirable film characteristics. Backing plate materials must also be compatible with the deposition system and with other operating conditions.
Engineers may wish to consider whether the supplier also provides target bonding, backing plates, custom machining and inspection as part of an integrated materials solution.
A supplier such as VEM is relevant in this discussion because thin-film production teams often need more than a raw material source. They may need sputtering targets, evaporation materials, bonding services, backing plates, crucible liners, and technical support aligned with the deposition process and application requirements.
Account for Film Uniformity and Material Utilization
Film uniformity is an important parameter for thin film deposition. Nonuniformity of thickness, deposit composition, and microstructure can degrade yield and cause downstream performance problems. Material selection is often a significant contributor.
For sputtering, target density, grain size, composition control, and erosion regime can directly affect how the material is removed from the target and how it is deposited on the substrate.
For evaporation, the size and shape of the material, its purity, and vaporization behavior can influence the stability of the coating process and the properties of the coating.
So important is this economic factor that a target which exhibits better usable life, better erosion behavior and less downtime may be cheaper overall, even if its initial cost for the amount of target material per unit area is higher.
Similarly, evaporation materials may mitigate chamber maintenance, rework and “downtime” if they vaporize cleanly and give consistent coatings.
Engineers should consider process economics rather than price per unit of target and/or evaporation material purchased.
Choose Materials for Electrical, Optical, and Mechanical Performance
Thin films are chosen as they impart a specific property conductivity, resistivity, reflectivity, transparency, hardness, adhesion, corrosion resistance, magnetic behaviour, thermal stability. Whatever physical substrate you use has to fulfil these requirements.
Aluminium, copper, titanium, tungsten, tantalum, nickel, gold, platinum, palladium etc can be used in conductive, barrier, adhesion and specialty films. Alloys and compounds can be rather more exacting.
Oxides, fluorides, ceramics, dielectric materials can be used in optical coatings, insulating films, transparent layers, protective surfaces.
Getting it right takes an understanding of the final device architecture as well as process specifics. A film that works in one stack may not do so in another if it induces stress, interacts with neighbouring layers or induces a moody material to protrude past it’s edge during processing.
Plan for Process Repeatability
In production runs, the right kind of repeatability matters as much as initial performance. Just as a material must work well in the first run, it must also perform in a similar manner across successive lots.
Achieving this means having a good source for procured materials, controlled manufacturing, reproducible specifications, and good communications between supplier and customer. Some important factors of repeatability would include:
- Chemical consistency from lot-to-lot
- Dimensional consistency
- Density consistency
- Surface finish consistency
- Package quality
- Predictable lead times
For custom targets specifically, and for specialty evaporation materials in general, teams need to make sure that the given supplier can actually support them with continuing production lots without changing any of the factors that are vital to process and performance.
Process engineers should not only have a document where good specification limits for a given material are agreed on, but they should also document inspection activities they performed along the way, and feedback on deposit performance.
Procurement teams, on the other hand, should resist the desire to substitute materials for price, unless they know from other experience just how even slight changes will affect deposition behavior.
Consider Cleaning, Reclaim, and Lifecycle Support
Thin film manufacturing very often requires more than just delivery of new materials. Every aspect of the chambers, shields, fixtures, crucibles and so forth can require cleaning, reclaiming, refurbishing, and perhaps even precious metal recovery when the materials being deposited may be relatively high value such as gold, platinum, palladium or other specialty metals.
A material supplier that understands both materials and process support can help teams achieve optimum uptime and lifecycle economics. Cleaning and reclaim services can support improved chamber performance, reduce waste and help teams manage the economics of expensive materials. In production environments, the services can be a component of an overall uptime improvement and operating cost control program.
When evaluating supply vendors for deposition materials, engineers and operations teams should consider whether a supplier can help not only with initial material delivery but also with ongoing deposition needs.
Avoid Common Material Selection Mistakes
There are several sins that can spell failure for a thin film deposition project. One of these is selecting a material based solely on chemical composition considerations, ignoring such factors as purity, physical form, density, geometry, bonding, and packaging.
Another is selecting a supplier without determining that the supplier knows the detail of the deposition method and tool in which the material is to be used.
Teams may miss check on crucible compatibility, target cooling requirements, backing plate specification, or cleanliness of the deposition chamber. Some teams simply use something that is correct, but the wrong form of the material for the deposition rate, film uniformity, and volume desired.
The most frequent sin of omission is concern only for the upfront cost; cheap materials can make an expensive deposit if they give rise to defects, yield loss, do not permit process stability, or shorten uptime. Materials for thin film deposition should be evaluated for the total process impact.
Final Selection Checklist
Before choosing thin film deposition materials or a supplier, engineers should review the following questions:
- Application: What function must the thin film perform?
- Deposition Method: Will the process use sputtering, thermal evaporation, electron beam evaporation, or another method?
- Material Composition: Which metal, alloy, oxide, ceramic, or compound is required?
- Purity: What purity level and trace element controls are necessary?
- Geometry: Does the target or evaporation material match the deposition tool?
- Bonding: Does the sputtering target require bonding to a backing plate?
- Thermal Behavior: Can the material handle the required power, heat load, or evaporation conditions?
- Cleanliness: How will the material be cleaned, packaged, and protected from contamination?
- Repeatability: Can the supplier maintain consistent quality across future lots?
- Support: Does the supplier offer technical guidance, custom materials, bonding, backing plates, crucible liners, cleaning, or reclaim services?
- Cost: Has the team considered the total process cost rather than only the purchase price?