Substrate Pretreatment Improves Film Performance for Flexible Electronics
By Kerry Cunningham
The potential applications for flexible, curved and foldable displays and electronics made with thin-film manufacturing techniques seem limitless. Want to roll up your touch screen monitor and put it in your suitcase? How about installing high-efficiency curved solar cells for optimum sun exposure all day long? Maybe you’d like to fold your paper-thin tablet so it fits in your pocket?
Such applications aren’t quite here yet, but incremental progress in substrate preparation, adhesion and pretreatment are important steps in making flexible electronics—devices for innovative display, wearable, biomedical, security, entertainment, aerospace, defense and other uses—a reality (see figure 1).
Figure 1. Flexible electronics enable displays with unique form factors. The figure shows some of the display-types that may be built with roll-to-roll (R2R) manufacturing technology for thin-film-based components.
A key enabler for the economic production of these systems is roll-to-roll (R2R) manufacturing technology for thin-film-based components such as touch screens for displays, barrier layers and flexible hardcoats.
In R2R manufacturing, rolls of substrate material up to several kilometers long are processed in a continuous fashion at high-speed. R2R combines the advantages of inexpensive, lightweight and flexible substrates with high-throughput capabilities. It also offers significant cost-reduction opportunities versus batch processing in terms of lower tool capital costs, better-utilized substrates and more economical process gas flows.
The state of the art in R2R technology has advanced to where it is possible to produce both electronic and optical devices with <40nm feature sizes on 50μm-thick polymeric substrates, thanks to advances in device patterning, chemical vapor deposition (CVD) sputtering, wet and dry etching, substrate cleaning and particle management.
Two key requirements for optimum R2R results are choosing the appropriate substrate and effective pretreatment of the substrate surface.
CHOOSING THE RIGHT SUBSTRATE
Thin-film devices can be deposited on many substrates, including stainless steel, flexible glass and some polymeric films, but not all flexible substrates can be used with R2R technology and not all flexible architectures require R2R processing. Thus, the design and development of flexible devices requires a thorough investigation of a potential substrate’s properties for a given application.
In general an ideal R2R substrate is smooth, optically transparent for displays, flexible and rollable, low-cost, resistant to chemical attack, dimensionally stable under thermal cycling, and able to act as an intrinsic barrier layer for low permeability to water and oxygen. Substrates 25–150μm thick are typically used in R2R processing, depending on the device architecture, process heat load, and desired mechanical properties.
The choice of substrate can be challenging, because each material offers different advantages and disadvantages, and no single material is accepted as the industry standard. For displays, the most promising commercially available materials are polyethylene terephthalate (PET), polyethylene napthalate (PEN), and polyimide (PI). See table 1.
Table 1. Typical substrates and associated properties used in flexible electronics. (Source: Applied Materials)
PET is a versatile thermoplastic polymer commonly referred to as Mylar® polyester film. PEN is a polyester with excellent thermo-mechanical properties combined with optical transparency and intrinsic barrier properties. PI is a polymer manufactured from a range of imide monomers.
PIs have high heat resistance and are often used where rugged organic materials are required, e.g., in high-temperature fuel cells, displays and various military uses.
PREPARING THE SUBSTRATE
No matter which material is chosen, substrate surface quality is critical to ensure that defects and particles aren’t present, because they can protrude through subsequent conductive or barrier layers. Surface roughness also plays a large role in device yields; thus planarization is required to attain yield and performance levels similar to those found on glass or silicon wafers.
Typically the substrate surface has to be pretreated. Surface moisture must be eliminated to minimize dimensional changes that may occur from moisture-induced swelling during device manufacture (see table 2) and to reduce the impact of outgassing on the deposition of intrinsic semiconductor materials.
Table 2. Water vapor and oxygen transmission rates of various materials. (Source: IDTechEx)
In addition, both the surface chemistry and surface energy of the substrate must be modified to increase the adhesion of inorganic materials deposited on it. Consequently, plasma pretreatment is required.
However, Applied Materials has found that when the substrate material passes through the process chamber at several meters per minute, existing plasma pretreatment methods are inadequate because the substrate is only in front of the pretreatment source for a very short time. Therefore, more powerful plasma-exposure techniques must be considered.
PLASMA PRETREATMENT OPTIONS
Table 3 compares three plasma environments that can be used for substrate surface treatment: DC glow discharge, mid-frequency (MF), and RF linear ion source (LIS) pretreatment.
Table 3. Performance characteristics of surface pretreatment options.
Often-used DC glow discharge pretreatment is similar to magnetron sputtering (see figure 2) except that a low magnetic field strength is used to confine plasma electrons and weakly enhance ion current density. But this limits the pretreatment effect.
Figure 2. Schematic of DC glow discharge process.
MF plasma pretreatment (see figure 3), meanwhile, provides higher ion energies and ion densities to promote better adhesion. MF treatment can be bipolar- or classically AC-driven, permitting operation with a broad range of reactive pretreatment chemistries by preventing buildup of charge on the target surface.
Figure 3 shows the adhesion performance improvement on both PET and BOPP substrates using MF plasma pretreatment. (BOPP is a biaxially oriented polypropylene film.) The bond strength for aluminum (Al) on PET improves from 3.8 N/inch to 5.2 N/inch at 45kW MF power, and from 1.0 to 3.1 N/inch for Al on BOPP with 45kW MF power.
Figure 3. The adhesion strength of aluminum (Al) on PET and BOPP polymer substrates improves with increasing MF plasma pretreatment power levels.
Figure 4. An RF linear ion source (RF LIS) installed in an Applied Materials SmartWeb® XL roll-to-roll (R2R) sputter system process chamber.
The third plasma pretreatment option, RF LIS, further improves adhesion, film density and barrier performance by modifying both the substrate surface and subsurface regions to increase the reactivity of chemical bonds between them and the process gases (see figure 4).
APPLIED MATERIALS SMARTWEB® XL SPUTTER SYSTEM
One of the industry’s most advanced R2R tools is the Applied Materials SmartWeb® XL R2R sputter system (see figure 5). It comprises a variety of different web-handling/ coating technologies and platforms for high-volume R2R production of barrier layers, touch panel devices and display antireflective stacks.
Figure 5. The figure is a schematic diagram of the winder/unwinder in Applied Materials’ SmartWeb® XL R2R sputter system for manufacturing flexible electronics and optical multilayers.
It features unwinder, process and rewinder modules that can be configured differently to meet application requirements. For example, each process module can be equipped with either high-rate planar or rotatable cathodes in separately pumped process compartments. The number of process modules required is determined by the layer stack architecture to be fabricated and the required tool productivity.
The standard Applied Materials SmartWeb® XL R2R sputter platform employs a tempered coating drum to accurately control both substrate temperature and planarity within the coating zone. This configuration also permits the use of a very small gap between the coating drum/substrate and the chamber separation walls, resulting in a large pressure separation (>2 orders of magnitude) between individual process compartments within each process module. This large separation reduces potential interference between the compartments.
The Applied Materials SmartWeb® XL R2R sputter system.
Coating uniformity is a key advantage of R2R processing. In batch coating systems the coating thickness and uniformity must be optimized in two dimensions, often leading to an increase in cathode design complexity and cost. In an R2R tool, however, the uniformity only needs to be optimized in a single dimension, simplifying tool design and reducing cost per unit area.
Furthermore, because the R2R process is continuous, the process gas and substrate material are used more efficiently and economically, and no significant substrate-to-substrate gas stabilization time is required as with batch processing.
Substrate characterization and pretreatment of the substrate surface are needed to eliminate surface moisture and minimize dimensional changes that may occur from moisture-induced swelling during device manufacture. They are also needed to reduce the impact of outgassing on the deposition of intrinsic semiconductor materials.
In addition, plasma pretreatment increases the adhesion of deposited inorganic materials by modifying the surface chemistry and surface energy of the substrate. This substrate preparation prior to deposition is key to improved film performance.
With good device performance, favorable production economics and enticing potential applications, R2R manufacturing of flexible electronics remains a goal that is moving closer to reality.
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