SU-8 Molds to Fabricate Semiconductor Devices
What Silicon Wafers are Used in SU-8 Molds?
A graduate research student used the following Silicon Wafers to creat SU-8 Molds for their PDMS Microfluidic Device Research.
Get Your Quote FAST!
SU-8 Photoresist, SU-8 Molds, Au-Epoxy Masters, and HMDS
This article will discuss SU-8 photoresist, SU-8 molds, Au-epoxy masters, and HMDS. It will also cover HMDS vs. SU-8. There are some differences between the two materials. But which one is right for your etching process?
SU-8 photoresist
SU-8 is a photoresist used in the fabrication of semiconductor devices. Its processing is similar to that of other negative photoresis materials. It requires temperature control during the baking steps. The baking times depend on the layer thickness and are longer for thicker layers. Keeping the temperature low is important to reduce stress formation in the thick layers, which can result in cracks.
The SU-8 mold is fabricated on a silicon wafer. The SU-8 master is then placed in a petri dish. Twenty grams of premixed PDMS polymer is then poured over the master. The petri dish is placed in a convection oven set at 65 degC for 12 h to cure. The mold can then be used to cast PDMS biochips.
The SU-8 photoresist has a high maximum absorption of 365 nm ultraviolet light. However, it cannot be exposed to g-line ultraviolet light. Its long molecular chains cross-link under UV light. The SU-8 photoresist is a solid-phase photoresist with high thermal and photon resistance.
SU-8 photoresist is an epoxy-based negative photoresist that contains parts that are cross-linked and soluble. This makes SU-8 ideal for imaging near the vertical sidewalls of thick films. Its high opacity makes it a good choice for a variety of applications, including lithography and microfluidic chips.
When using SU-8, it is essential to clean the wafer before applying the photoresist. It is best to clean the wafer with piranha solution or acetone if the room is clean. Then, it is important to heat the wafer to remove any moisture on its surface. A 15-minute heat period at 120degC is recommended as this will make SU-8 adhere to the silicon much better.
SU-8-Si masters
There are various techniques to observe the profile of a structure using SU-8-Si masters. One technique requires dicing the silicon crystal, cleaving it into two planes, and mounting the pieces on a customized vertical holder. Another technique is bright-field microscopy.
SU-8 has good adhesion, but its adhesion depends on the material. It is weaker when used with gold, while it is good with silicon with native oxide and silicon nitride. It is also affected by the chemical, lifting off with KOH while sticking with HF. The cleanliness of the substrate is also important. MCC claims that the new SU-8 2000 series has better adhesion because the solvent used in the production process is different.
The SU-8-Si masters used for this process are made of thin layers. The layer height can be up to 25 mm. The thickness of the layers controls the transition to the planarized regime. The thickness of the overlay is proportional to its height and width. Then, a second thin layer is added on top of the first layer to create a rounded effect on top of the underlying rectangular patterns.
The multiple-layer SU-8 coating provides planarity and surface quality, as well as low ohmic losses. The soft and post-bake times of SU-8-Si masters are calculated by using a systematic procedure. A double corrugated waveguide has been fabricated as a test structure. It is designed to operate at 0.3 THz.
The SU-8-Si masters were used for optical and mechanical characterization. They are also useful in microelectronics.
SU-8-Si vs Au-epoxy masters
SU-8-Si and Au-epoxy masters exhibit the same surface properties, and both masters provide a good choice for 3D printing. In the case of SU8, the material also exhibits better mechanical properties. The difference between the two masters lies in the way that they are processed.
SU8-coated glass has similar behavior to neurons grown on glass. Various treatment protocols of SU-8 were evaluated in order to obtain optimum biochemical, mechanical, and electrical properties. Thermal annealing, which induces partial cross-linking of SU8 before gold deposition, was found to give the most stable electrode properties. Stability was monitored using optical microscopy and four-point probe sheet resistance.
SU-8-Si and Au-epoxy masters are widely used in the fabrication of microelectronic devices, but its application in biosensing has been relatively overlooked. The epoxy-based negative photoresist's photoluminescence hinders fluorescent labelling of biorecognition events, but it can be used as a sensing transduction parameter. This makes SU-8-Si a valuable tool in immunosensing, as it reduces with modification of surface chemistry and attachment of an antigen-antibody pair.
SU-8-Si masters are used in high-end micro-electronics manufacturing. These materials exhibit higher resolution, easier handling, and better surface chemistry than their counterparts. They also demonstrate better thermal stability and tensile strength.
SU-8-Si films have a good adhesion to gold. The two masters were prepared by using different procedures. For SU8-Si masters, the first step of the process involves the application of adhesive tape to the surface. The second step involves the removal of the tape. This step was repeated 50 times until delamination occurred.
HMDS
The SU-8 mold is manufactured on a silicon wafer. It is used for casting PDMS biochips. These chips are used in the study of cancer cells. The process is known as photolithography. SU-8 molds are widely used in this process. The process flow is shown in Figure 3.6.
The application of SU-8 is performed using a slow spin. It should be rotated at least 20 rpm. For two-inch wafers, the rpm should be increased. The process can be optimized by adjusting the SU-8 mold design. One of the most important parameters for SU-8 molds is the PEB temperature. Increasing the PEB temperature can prevent cracks and improve the resist profile and contrast.
The temperature of the SU-8 mold should be between 230 and 300degC. Otherwise, the layer will detach from the mold due to mismatched coefficients of thermal expansion. The temperature is then decreased to room temperature. The final process is called the soft bake. The process is followed by a lithography development process.
The SU-8 mould is released after the sacrificial titanium layer is etched using HF. The resulting silicon nanochannel has a final dimension of 379 nm by 40 nm. A thick SU-8 mould enables better pattern definition and improves the efficiency of forming thick deposited structures.
When SU-8 is crosslinked, it is very difficult to reflow. At 210degC, it cannot reflow after 21h. At 220degC, it starts to change colour and becomes black.
T-topping with SU-8
SU-8 molds are ideal for fabrication of high aspect ratio structures. For example, they can be used for biomedical diagnostic sensors and as molds for soft lithography. In addition, they can serve as precursors for glass-like carbon electrode arrays and carbon-electrode dielectrophoresis. In such cases, the narrow gaps between individual electrodes are a requirement. Therefore, proper processing of SU-8 molds is essential to achieve the desired narrow gaps.
When evaluating the effects of SU-8 molds, it is important to determine the minimum thickness of overlay before executing the process. This thickness ratio influences the transition profile and the height of the overlay. The transition height can be further divided into two stages: a steep increase followed by a plateau.
The photoresist behavior of SU-8 molds changes as the pre-exposure bake progresses. As the process progresses, the SU-8 photoresist may develop cracks or delamination layers. In order to suppress these effects, the photoresist is heated to 120 degC. The process is similar to the soft bake except for the temperature.
To make SU-8 molds with SU-8 molds, a flexible substrate is used. This flexible substrate can be patterned with SU-8 by photolithography and used as a carbon micromold. Earlier approaches have used rigid substrates coated with a sacrificial material. The SU-8 photopatterned layer is then chemically dissolved, releasing the topography.
The final release of SU-8 molds is long and the final release time increases with the footprint. However, it is possible to peel the mold after developing with a flexible substrate.