Substrates for Microsystems

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What Substrates are used to Fabricate Microsystems?

Microsystems are small mechanical and electrical devices that are typically built on a substrate material, which provides a foundation for the microsystem components. There are several types of substrate materials that are commonly used for microsystems, including:

1. Silicon: Silicon is a popular substrate material for microsystems because it has a high melting point and a low coefficient of thermal expansion, which makes it resistant to thermal stress. In addition, silicon is an excellent conductor of electricity and has a high surface area, which makes it suitable for use in microelectromechanical systems (MEMS).

2. Glass: Glass is a transparent substrate material that is commonly used for microsystems that require optical components, such as sensors and display devices. It is also chemically resistant, which makes it suitable for use in harsh environments.

3. Plastic: Plastic is a lightweight and flexible substrate material that is commonly used for microsystems that require a high degree of conformability, such as wearable devices. It is also relatively inexpensive and easy to work with.

4. Ceramics: Ceramics are hard and brittle substrate materials that are commonly used for microsystems that require high strength and durability, such as aerospace and defense applications. They are also resistant to high temperatures and chemical attack.

5. Metal: Metal is a strong and conductive substrate material that is commonly used for microsystems that require high electrical conductivity, such as electronic devices. It is also resistant to corrosion and has a high melting point.

 

Substrates Used to Fabricate Microsystems

Substrates used to fabricate microsystems come in many shapes and sizes, depending on the application. They can be flexible or rigid, and can be manufactured from materials such as glass, polystyrene, fused-silica, or conductive SU-8.

Flexible substrates

Compared with rigid substrates, flexible substrates are lightweight, thin, and heat-resistant. They are also easy to install and maintain.

As a result, they are increasingly being used in PCBs. Their unique properties provide them with a wide range of applications. Among them are PCBs for power electronics, sensing technology, and memory power.

Flexible PCBs have high capacity and can be inserted into narrow slots. This allows for the use of small gadgets. Moreover, they can be manufactured with minimal errors. Consequently, they are ideal for applications requiring high-performance.

Aside from these advantages, flexible substrates also offer a sleek appearance. Moreover, they are highly resistant to heat, acid, and UV rays. In addition, they can be easily repaired. These features make them ideal for medical devices. Besides, they are easy to integrate with computing technology.

The development of new microfluidic technologies on flexible substrates has been a major area of focus for research. There has been a lot of innovation in the field over the last decade.

However, there were some difficulties in the early stages of the development of these miniaturized analytical platforms. These problems were overcome through the implementation of several steps. Some of these processes were micromachining and microcontact printing.

Today, flexible PCBs are becoming the preferred choice of manufacturers. Thanks to their low cost, reliability, and sleek appearance, they are gaining acceptance in the industrial and medical sectors. Similarly, they are expected to play an important role in the development of driverless cars and drones.

Using flexible substrates to manufacture microfluidic micromachines is a promising trend. This means that they will become a key emerging area in the next decade. It has been shown that they could reach $10.5 billion by 2022.

Glass

Glass substrates are used to manufacture microsystems, such as microfluidic chips. They are often fabricated by a process called nanoimprint lithography. These substrates are characterized by a convex surface that allows less dust to enter the assembly.

Other substrates used to fabricate microsystems include hermetically packaged devices and substrates that are supported by silicon substrates. In one particular application, a glass substrate has been laminated with a silicon substrate. The two surfaces are pasted together and a cutout is incorporated.

This process creates a hermetic seal that can withstand high temperatures. The silicon and the glass have been bonded with a resin that can endure harsh conditions.

One example of the aforementioned process is the use of a temporary bonding technique developed by Corning. This technology allows for the handling of thin glass substrates in a high volume production environment.

Another way of achieving the same results is to use a wet etching process. Wet etching is widely used in the fabrication of microsystems. This process involves the use of high-energy vapors to scavenge SiO2 from the glass.

As you can see, there are many different techniques for creating glass substrates. It is important to determine which technique best suits the application in hand. Among the available options, anodic bonding and wet etching are the most common methods.

Another technique is to produce a light-shielding film on the main surface of the substrate. This feature is especially useful for enhancing the amount of reflected light. Light-shielding is often considered an important design feature because it helps increase the yield of a glass substrate.

Other possible processes for creating a Glass Substrates include anodic bonding and wet sandblasting. Both of these methods produce reliable structures that can be leveraged by the end user.

Fused-silica

Fused-silica substrates are used to fabricate microsystems for various applications. These can include sensors, actuators, and resonators. Several different methods have been described for structuring fused-silica substrates. However, there is still room for improvement in the methods. Using a new process, the surfaces of these materials can be smoother, which may significantly improve the optical performance of these devices.

The process begins with patterning a silicon wafer. A masking layer is then deposited, which may be made of Si or nickel. This etch mask is then removed using a wet etchant.

A second silicon wafer is then bonded to the first one. The silicon wafer can then be etched to produce a structure with the desired shape. For example, the structure shown in the drawing is a microfluidic chip.

Alternatively, the structure can be a microfluidic meander. It can also be an out-of-plane vibration mode device.

Other elements that may be bonded to the microsystem are capacitors, active devices, and resonators. Electrically insulating materials such as sapphire, Zerodur, or ceramic can be incorporated to insulate the layers. In some cases, glass frit is used to bond the layers.

The substrate is then annealed at a temperature between 1200 and 1400 deg C. The annealing temperature is controlled to reduce DRIE sidewall scalloping. Depending on the application, the temperature may vary. Nevertheless, the thickness of the silica shell is a critical point. Generally, a thickness of several mm is recommended. Simulations show that the thickness is sufficient to prevent mode migration into the silicon core.

Bonding can then take place between the fused-silica substrate and the layers that form the MEMS device. One way of doing this is to deposit a high-phosphorus doped polysilicon layer on the substrate. Doping the polysilicon allows it to act as an electrode to close the gap between oxide layers.

Polystyrene

Polystyrene substrates can be used for making microsystems such as microfluidic devices. Such a polymer is solid at room temperature but becomes rigid when cooled. It can be molded or cast into fine detail.

Polystyrene is a moderately strong polymer that can be crosslinked by divinylbenzene. However, it is not biodegradable. In order to reuse the polymer, a recycling process is needed. The Society of Plastics created a resin identification code for polystyrene. Currently, the polystyrene used for packaging is not recycled in curbside collection programs. Using a polystyrene substrate in a recycling process is one way to reduce the impact on the environment.

Polystyrene is used in many products and devices. For example, it is a main ingredient in the synthesis of new plastics. Several other applications include injection molding, plastic model assembly kits, license plate frames and disposable plastic cutlery.

In order to modify the surface of the polymer for device fabrication, orthogonal functionalisation schemes or orthogonal functionalizations are needed. Here, we propose an approach to achieve such surface modification without using conventional nanolithography. This involves the miniaturization of hot-embossed prestressed polymer films. With this approach, the pattern size is reduced by 10x. Moreover, the shrinking process is controlled to ensure that topographical features are maintained and the aspect ratio is preserved.

Three sequential miniaturization cycles were carried out. The first cycle was based on etching the Si patterns on the substrate. To achieve a better selectivity, the SF6 flow rate was reduced to 40 sccm.

For the second miniaturization cycle, the Si intermediate master was used. This master provides a flat substrate surface and decouples the aspect ratio of the Si pattern from the shrink dynamics of the polymer film. Thus, it allows for precise structures with vertical sidewalls.

Conductive SU-8

SU-8 is a silicon-free photoresist that has been developed for microelectronic applications. It can be used for building microsystems that require high-resolution masks. These structures include strain sensors, microactuators, passive elements, and MEMS devices.

A conductive SU-8 layer can be formed by mixing silver nanoparticles with a conductive organic filler. This material has a high electrical conductivity and can be used for passive electronic devices such as inductors and resistor-capacitor circuits. In this study, we present a method to fabricate planar inductors using conductive SU-8.

The fabrication process consists of a three-step process. First, the conductive SU-8 is deposited on a SiO2 insulating layer. Next, the SU-8 layer is patterned with negative photolithography. Finally, the SU-8 layer is exposed to a UV irradiation. Using a low-UV exposure intensity, residual stress is prevented from developing in the SU-8 layer.

During the baking step, temperature control is important. Increasing the baking temperature must be gradual. If the rise in temperature is too rapid, cracks may form.

Another important step is the use of a protective layer. The protective layer improves the adhesion of the conductive traces to the substrate. This technique also minimizes the minimum bending radius. Moreover, it reduces the likelihood of delamination.

We also demonstrate how we can fabricate passive elements using conductive SU-8. For example, we build bilayer microactuators with a beam width of 150 mm and a layer thickness of 220 nm. As we mentioned earlier, the electrical conductivity of a composite material is dependent on the amount of conductive filler and the proportion of filler. With this method, we can create a broad map of composites for different applications.

Besides flexible substrates, this technique can be used to pattern high-density interconnection lines. This technique has the potential to create microsystems that can rival the flexible PCB industry.