These jets cover the entire width of a field.
Enhancing materials for hi-res patterning to advance microelectronics
Further, these inkjet heads have been demonstrated to perform reliably without defects such as missing or deviated nozzles, loss of volume control, or loss of placement accuracy in excess of 6 months on J-FIL steppers. Imprint materials, including the imprint resists and the substrate adhesion materials, are important building blocks of the J-FIL stepper process. The primary function of the imprint resist is to form a polymer replica of the template. This replica forms a sacrificial patterned material that serves as an etch mask during subsequent reactive ion etching that transfers the pattern to an underlying film or film stack on the wafer.
The adhesion material serves to modify the surface properties of a wafer to promote wetting during the liquid phase prior to UV curing. It also promotes adhesion of the imprint resist to the wafer during the solid phase after UV curing. The development of these materials is discussed next.
The imprint resist is a material composition comprising of several constituents designed to achieve desired process considerations. Some of the process considerations can place conflicting requirements on the composition requiring a good understand of the trade-offs. The imprint resist is dispensed as discrete drops using inkjets in a liquid formulation, the liquid is captured in the template features, and finally the liquid is cross-linked using UV light to polymerize into a solid material. Hence, the resist must have desired liquid and solid phase properties. The liquid phase properties include density, viscosity, surface tension, contact angle, and volatility.
On the other hand, the solid properties include cohesive yield strength, shear strength, elongation to break, adhesion to substrate, and ease of release from template. The optimum material formulation involves a consideration of all these characteristics. The liquid properties of the resist are primarily tailored towards robust inkjettability of picoliter volumes, low parasitic evaporation, and fast defect-free wetting of both the substrate surface and the template features.
Hence, fluid viscosity and evaporation properties need to be carefully balanced. When all template patterns are fully filled, the UV cross-linking step follows. During UV exposure, the material needs to cross-link quickly for high throughput and to prevent thermal loads on the substrate and template.
Thermal loads can compromise overlay by causing parasitic expansion and nanoscale in-plane distortion of the patterns. Next, during the separation step, it is necessary to ensure that the material adheres to the substrate and not to the template, which is a conflicting requirement when compared against the liquid phase property of requiring wetting and filling of the template features. Adding polar resist components helps with these properties, but excessive amounts increases the surface tension and reduces the fill speed in the fluid phase.
Many types of UV curing chemistries including methacrylate, epoxy, vinyl ether, thiol-ene, and acrylate have been considered for imprint resist. Methacrylate and epoxy have considerably slower polymerization rate. Hence, they are not practical for high-throughput volume manufacturing.
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Most of the UV imprint materials are acrylate-based materials. Vinyl ether, which has low viscosity, relies on cationic polymerization to cure the liquid resist. It is inherently not inhibited by oxygen, which scavenges radicals. On the other hand, the cationic initiator is sensitive to the presence of excessive moisture and has concerns related to shelf-life stability as well as interaction with fused silica templates. Thiol-ene provides a promising alternative to acrylate chemistry for UV imprint resists. It has distinct advantages over acrylate systems including low oxygen inhibition during cure and low separation force against fused silica template.
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The disadvantages of thiol-ene systems are strong odor of thiol, and limited amount of commercially available compounds. The shelf-life of thiol-ene systems can be extended by adding a stabilizer. In summary the leading choice of polymerization chemistry for imprint resists is acrylate materials.
Acrylate materials possess good curing speed, and are typically one order of magnitude faster than methacrylate chemistries. It is also advantageous that many different acrylates are commercially available, which readily enables testing of many formulations of imprint resists with varying parameters such as viscosity, surface tension, and solid mechanical properties. Acrylates also have excellent shelf-life lasting at least 6 months, which can possibly be extended to 12 months. The primary limitation of acrylates is that it is susceptible to oxygen inhibition, which needs to be balanced.
Early resist formulations included silicon to enable high etch resistance in oxygen However, silicon containing materials, if trapped in the template during a process excursion, can cause template cleaning issues as they form silicon dioxide easily in small features and are then hard to remove chemically. Later resist formulations therefore used silicon-free organic materials 64 , 91 , and used the film stack shown in Figure 9.
J-FIL material stack showing silicon-free organic resist and adhesion layer.
The fidelity of the J-FIL stepper process is influenced by many solid phase resist characteristics including preferential adhesion ratio defined as the ratio of adhesion between the resist material and the adhesion layer A 1 on the substrate divided by the adhesion between the resist material and the template surface treated with a release layer A 2.
This RIE step has a strong physical component that can damage the top of the resist feature. To maintain a good resist mask after the break-through etch, the starting resist height h needs to be 2—3 times the thickness of the underlayer residual and adhesion layer combined. The adhesion layer is typically deposited onto the substrate via spin coating or vapor treatment methods. The adhesion layer also needs to be engineered in such a way that it can work on typical substrates such as Si, SiO 2 , SiN, and so on, and be readily wettable by the imprint resist in its liquid form.
The rest of the risks are addressed below. The strategy chosen for J-FIL is one where a template replica can pick up a small number of repeaters as long as the total defectivity stays below what is expected in memory applications. This is a significant achievement as the particle levels are 2—3 orders of magnitude better than best practices, even in the semiconductor fabrication domain.
This tool is capable of identifying particles that affect 2x nm memory device patterning A multifaceted particle control approach, both in the tool and in the resist materials, has resulted in this capability. Particle reduction in the resist, down to 0. In the tool, particles can result from air flow, from ceramic material surfaces, and from moving components, particularly the ones involving frictional contact. A detailed treatment of this topic is included in Ref.
Two important aspects of a viable J-FIL stepper process are described next. First, the need for a thin and uniform residual layer is discussed. It is important to note that when the template engages with the resist monomer fluid drops, the local fluid flow is a function of the template patterns. Grating structures can cause highly anisotropic flow patterns, while dot patterns result in isotropic flow.
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Ideally an inverse optimization scheme that is based on comprehensive forward models, is needed to correctly identify the volume and placement of the monomer drops. Heuristic algorithms have found success in addressing this problem 71 , while more comprehensive model-based approaches have been pursued 94 with some success. Illustration of template and wafer with the resist fluid between them, wherein the advancing front of the fluid approaches the mesa corner M c. Next, methodologies used to avoid fluid extrusions beyond the full rectangular field and partial fields are explained.
Precision fluid confinement at the field boundaries is needed to enable patterning of precise field rectangles with no fluid extrusions outside these rectangles. This region provides a buffer zone that tolerates significantly less precise definition of the liquid front—for example, tens of microns of variation—as compared to the zero-gap requirement between fields. In addition, pinning similar to the one shown in Figure 11 can also be achieved at the wafer edge by having a sharp transition in cross-section profile on the wafer side.
Enhancing materials for hi-res patterning to advance microelectronics
As discussed in the literature 95 , 96 , there are two major types of silicon wafer edge cross-section profiles: i blunt-nosed rounded edge; and ii bullet-shaped beveled edge. The latter includes a sharp corner at the onset of the beveled surface and can cause liquid pinning. Additionally, the wafer edge needs to be prepared to avoid any edge beads resulting from thin film deposition processes that result in raised regions at the edge of the wafer as this will invariably cause the template to interfere with this edge bead causing disruption of the edge field patterning process.
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As shown in the right image of Figure 12 , four fields can be printed with zero gap between them and without void defects voids are seen in the middle image of Figure Such small variations do not appear to affect subsequent etch processes. Figure 3 depicts the steps in the J-FIL process.
In Step 3, the micro-scale template deformation causes high in-plane overlay errors making it very difficult to start the overlay process. Therefore, after the template has relaxed back to its nominally flat form in Step 4 of Figure 3 , overlay correction is initiated. First, the i-MAT system provides eight independent alignment errors x and y errors near the template corners.