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This edition of Flat Panel Focus^® offers relief for your film stress—solutions for your bent or broken substrates and peeling films. Ken Nauman also answers reader questions about arc-management setup, banding, and output cable choice.

Film Stress Relief

Excessive film stress can cause problems ranging from cracking (crazing), wrinkling, curling, and peeling films to bent or shattered substrates. Even when the results are not this extreme, film stress can dramatically reduce device lifetime. This is particularly a problem for the large (3 m x 3 m) and thin (< 1 mm) substrates that are typical of FPD manufacturing. Vertical deposition techniques may exacerbate stress-related problems, and film stress is a critical issue to address for flexible substrates. Although the potential problems are many, the good news is that it’s possible to control stress and achieve the film properties that you require.

There are two primary types of stress where thin films are concerned: thermal and intrinsic.

Thermal Stress

Thermal stress is the result of the difference, during cooling, in contraction rates of the deposited film and the substrate (coefficient of thermal expansion or TCE). If the film contracts more than the substrate, the substrate will likely bend in a concave formation as its larger surface area is drawn in toward the film’s smaller surface area. On the other hand, if the substrate contracts more than the film, the substrate will likely bend in a convex formation. This is most often a problem for processes in which the device cools after a film has been deposited at elevated heat.

Unfortunately, there is nothing you can do about the inherent reaction of a material to heating and cooling. However, a few possible solutions exist. One is to switch to a different deposition method. Evaporation, for example, can be performed at lower substrate temperatures and thus produces thermal-stress-free films. However, it’s associated with possible adhesion (Figure 1) and film density problems. Another solution is to switch to substrate and film materials with similar coefficients of thermal expansion. Also consider the use of RF-superimposed DC, which is a newer technique that can be performed at lower temperatures. It produces results that are comparable to high-temperature processes.

Figure 1. Adhesion test showing proper adhesion (left) and poor adhesion (right)—The film on the right was deposited by evaporation, which often is associated with adhesion problems

Intrinsic Stress

Every material, in its native form, has an inherent molecular structure that gives it certain tendencies in terms of film stress. However, the material’s inherent properties can be influenced by controlling ion energy in order to “tune” this stress. An ion with low kinetic energy impacts the substrate with a weak force, producing low packing density. In effect, the deposited atoms are too few and far apart, causing forces of attraction across the empty spaces between them. This creates tensile stress, in which the film seems to shrink up. If the film is deposited directly on the substrate, this likely produces a concave curvature (Figure 2). If it is deposited on top another film, the likely result is poor film adhesion.

On the other end of the spectrum, ions with high kinetic energy come down very forcefully into the deposited film, producing high packing density. If ion energy is too great, the deposited atoms may be packed too tight, causing them to exert a force of repulsion against each other. This results in compressive stress, which can be strong enough to bend the substrate backwards (Figure 2). Like tensile stress, compressive stress can also result in adhesion problems for films that are sputtered on top of another deposited layer.

Figure 2. Tensile and compressive stress—Insufficient ion energy can produce tensile stress, in which the film seems to shrink. This can result in concave curvature of the substrate (left). Excessive ion energy can produce compressive stress, in which the film seems to expand, possibly resulting in convex curvature of the substrate (right)

Ion Energy Management (IEM)

There are many factors affecting ion energy and thus film stress, including pressure, source-to-substrate distance, and process power choice.

Pressure
The term mean free path refers to the average distance a particle can travel without collision. At higher pressures, more particles are present per cubic centimeter than at lower pressures; mean free path is shorter (Figure 3) and more collisions occur. Collisions reduce the kinetic energy with which atoms impact the substrate. The result is lower packing density and greater tensile film stress.

On the other hand, if pressure is too low and mean free path is excessive, the lack of collisions may result in over-energized atoms that impact the substrate with too much force. This may produce excessive packing density and compressive stress.

Figure 3. Fewer particles are present at lower pressures; mean free path and ion energy are greater. At higher pressures, more particles are present; mean free path and ion energy are lower

An equilibrium pressure must be achieved to produce the ideal film stress properties for any given process. Because film stress is so highly influenced by pressure, a good rule of thumb is to adjust pressure only to control film stress and not to manipulate sputter rate, voltage, or any other process condition.

Figure 4 shows the general relationship between pressure and tensile/compressive film stress. The process of determining the ideal pressure for your process involves trial and error. For example, if you’re experiencing tensile stress in your films, try turning down the pressure about 0.5 mTorr to increase ion energy. If the tensile stress remains, turn pressure down another 0.5 mTorr. In cases of compressive stress, try increasing pressure. In either case, experiment until film stress properties reach an acceptable level. Keep in mind that if the plasma extinguishes, the pressure has been turned down too low. If the plasma becomes unstable and arcing increases, the pressure has been turned up too high.

Figure 4. General relationship between pressure and film stress

Pressure Uniformity
It’s also important to have uniform pressure throughout your chamber. If different pressure zones exist, film stress will vary across the substrate. This also makes it very difficult to adjust pressure in order to achieve specific film stress properties. Figure 5 shows a chamber with extreme pressure gradients that might result in varying film stress properties across the substrate.

Figure 5. Pressure gradients resulting from system design issues can produce varying film stress properties

To improve pressure uniformity, it may be necessary to add gas manifolds or modify existing ones. Figure 6 shows a design that produces excellent pressure uniformity and thus enables better control over film stress. It allows each MFC to be adjusted independently in order to compensate for chamber design and cathode design issues. Figure 7 shows modifications that can be made to the manifold itself.

Figure 6. Schematic of a segmented gas manifold

Figure 7. The size of and space between a gas manifold’s holes can be adjusted in order to improve uniformity

Source-to-Substrate Distance
Source-to-substrate distance influences two crucial phenomena: plasma ignition and film stress. An airplane must have a certain length of runway in order to achieve the speed necessary for takeoff. Similarly, an electron must have sufficient space (mean free path) to accelerate across in order to create a collision powerful enough remove an electron from an atom. When enough ions are created, a cascade effect ignites the plasma and deposition can begin.

Similarly, atoms of target material must have a specific amount of kinetic energy so that they impact the substrate neither too forcefully (results in compressive stress) nor too feebly (results in tensile stress). Source-to-substrate distance defines the length of the “runway” for atoms of target material. A shorter runway produces lower kinetic energy, while a longer runway results in higher kinetic energy.

For every process, there is a source-to-substrate distance that produces the ideal amount of kinetic energy. In general, this usually falls somewhere between 5 to 15 cm (2 to 6″). Determining the best distance for your process is usually a process of trial and error. Start at 10 cm (4″) and increase or decrease the distance depending on the results and your desired packing density/film stress properties.

Process Power Selection
Certain types of process power produce more highly energized ions than other types. For example, pulsed DC and AC produce higher ion energy and packing density than straight DC. Figures 8 and 9 show how the film density produced by straight DC compares to that produced by pulsed-DC power. Please note that DC is an excellent option for many processes, but if you need to increase packing density, consider adding pulsing or switching power types.

Figure 8. Film quality produced by straight-DC power—Lower ion energy produces lower packing density and poor film uniformity
Source: Centre for Advanced Materials and Surface Engineering, University of Salford, U.K.

Figure 9. Film quality produced by pulsed-DC power—Higher ion energy produces greater packing density and good film uniformity
Source: Centre for Advanced Materials and Surface Engineering, University of Salford, U.K.

If you are experiencing film stress problems, first try adjusting process pressure. If that doesn’t solve the problem, try modifying source-to-substrate distance. If you continue to experience film stress issues, consider switching process power types. Please also note that if you are using pulsed DC, you can adjust pulsing frequency and duty cycle to control ion energy and thus film stress.

Table 1 below provides general guidelines for process power selection, including ratings for packing density.

Table 1. Process power selection table

Of course, there are other factors to consider besides film stress when selecting a power supply. Please also consult the following resources for further advice on process power selection, and feel free to contact us for further assistance:

• The Art of Choosing the Right Power Supply, Q1 2007 Sputter Spotlight E-newsletter
• Choosing the Best Power Supply for Your PVD Process, Q2 2008 Flat Panel Focus E-newsletter

Other Film Stress Solutions

Cooling and Venting in Inert Gas

A relatively simple solution to reduce stress is to let devices cool as much as possible before venting the chamber. This prevents the newly deposited films from being traumatized by a sudden change in temperature and pressure. Such trauma seems to increase film stress. For the same reason, it also may be beneficial to vent in an inert gas such as argon. Nitrogen, a more affordable alternative, may also be used. This is very important for certain processes, such as the deposition of very thin TCOs, in order to prevent over-oxidation.

Post-Deposition Annealing

Intrinsic stress problems can sometimes be solved by annealing—heating up the film to relieve some of the material properties that cause this stress. This is done after the deposition process is complete to release some of the energy stored in the film. Of course, this is not an option for polymer substrates and color filters, which are sensitive to high temperatures. Also, thermal annealing and other high-temperature manufacturing processes can cause diffusion of underlying layers. Some manufacturers use laser annealing to overcome this problem, because this technique heats only the topmost layer.

Figure 10. Effect of temperature on film stress properties—Thermal annealing alleviates both tensile and compressive stress

 Periodically Adjusting Process Conditions/Film Characteristics

If process conditions remain exactly the same throughout the deposition of an entire layer, especially when that layer is particularly thick, undesirable film properties, such as film stress, may become more pronounced. Certain periodic process adjustments can serve to “re-boot” film properties. For example, slightly changing pressure approximately every 100 Å gives a fresh start that prevents either tensile or compressive stress from dominating. Even if no extreme film stress is evident, periodic modifications of film properties may prevent problems from developing.

Interleaving is a more extreme process that is based on the same principle. Layers of film in tensile stress are alternated with layers of film in compressive stress. The net stress of the entire stack, then, is neutral. The main drawback is that this only works to a certain film thickness before adhesion problems develop.

If you have further questions about film stress or need hands-on help solving film stress in your process, please contact us.

Ask the FPD Expert!