Inkjet etching of micro-via holes in thin polymer layers

Facilitated by the development of various direct-write techniques and functional polymeric materials including polymer based conductors and semiconductors, printed electronics are flourishing both commercially and as a research topic. This is not only because of their simpler manufacturing routes and lower cost, but also as a result of lower processing temperatures and better compatibility with flexible substrates, compared with conventional electronics. The development of conventional electronics has been guided by Moore s Law, the driver for which lies in the demand for electronic devices with better performance and portability at lower prices. Therefore, one can expect a similar trend for printed electronics to guide its development. Multi-layered printing can be adopted in printed electronics to achieve higher density integration, so that this development trend can be maintained. In such circumstances, creation of electrical connections between multiple layers emerges as an important issue for printed electronics. Inkjet-etched via holes are one potential solution to providing such electrical interconnections, and which can provide good integration with other inkjet-printed features simply by switching nozzles. This thesis aims to elicit a better understanding of the physics involved in inkjet etching and investigate the capability of the inkjet etching technique. In the thesis, the factors that can affect the size of via holes produced by inkjet etching are evaluated, which is significant for evaluating the capability of this technique to deliver industrially relevant features. Identified factors include droplet ejection frequency, droplet diameter, solvent properties and substrate temperature. Droplet ejection frequency, i.e. the reciprocal of the time interval between drops, determines the extent of evaporation of the solvent between two consecutive drop impacts. Droplet diameter determines the radius of the wetted area after the droplet I impacts on the surface and spreads into a sessile drop. Solvents with different evaporation properties result in different size evolution with the number of drops dispensed, as does droplet ejection frequency. Higher substrate temperatures can reduce the drop diameter during flight and decrease the evaporation time on polymer surfaces, which can shrink the size of via holes. Another important issue is achieving complete polymer penetration as residual polymer creates an electrical conduction barrier after such holes are subsequently filled with conductive materials or act as a barrier to filling by electroplating. Experiments have been carried out to test the effect of outer diameter and polymer thickness on polymer penetration. Electroplating is utilised to test the completeness of via hole penetration. A mechanism using the Marangoni effect to explain the protrusion drying pattern other than a hole in the polymer layer is proposed

The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing

This paper reports the penetration limit of via holes through dissolving dielectric polymer thin films by inkjet printing. It was found that both the outer diameter of via holes and the polymer thickness affect the penetration depth from the experimental results. Based on this finding, a more accurate relationship between the inner diameter of via holes and the diameter of in-flight droplets for different polymer thicknesses is obtained

The penetration limit of poly(4-vinyl phenol) thin films for etching via holes by inkjet printing

This paper reports the penetration limit of via holes through dissolving dielectric polymer thin films by inkjet printing. It was found that both the outer diameter of via holes and the polymer thickness affect the penetration depth from the experimental results. Based on this finding, a more accurate relationship between the inner diameter of via holes and the diameter of in-flight droplets for different polymer thicknesses is obtained

The effect of droplet ejection frequency on inkjet-etched micro via holes

Inkjet etching has been identified as a potential route to formation of micro via holes in polymer dielectrics. Such vias could facilitate three-dimensional integration and sequential build-up fabrication in printed electronics. In the research reported in this paper, ethanol droplets were jetted onto a poly(4-vinyl phenol) (also known as PVP or PVPh) layer at different frequencies in order to observe the effect of droplet ejection frequency on the diameters of the via holes produced. The results demonstrate that via holes remain the same diameter at a low drop ejection frequency, while they enlarge at a relatively high frequency. A mechanism for this behaviour is proposed for which high speed photography provides evidence

Direct-write techniques for maskless production of microelectronics: a review of current state-of-the-art technologies

Recently, there has been growing interest in direct-write methods for the manufacturing of microelectronic products, as the entire electronics industry sector is aiming towards low cost, rapid manufacturing and shorter time-to-market, as well as reduced environmental impacts. This paper will review the main direct-write techniques, most of which have been invented or seen significant development during the last decade. These techniques include droplet-based direct writing, such as inkjet printing, filament-based direct writing, such as the Micropen and nScrypt processes, tip based directwrite methods, and laser beam direct writing. For each category, only a few examples are presented, although there are a number of specific methods and variants within each of these categories

A mechanism of the penetration limit for producing holes in poly(4-vinyl phenol) films by inkjet etching

A penetration limit has been experimentally demonstrated for inkjet etching of holes in thin polymer layers. A mechanism combining the competing coffee ring flow, polymer dissolution and diffusion into the solvent drop, and the interaction between the contact line during evaporation and the softened deformable polymer, is proposed to explain the existence of such a penetration limit. The height-averaged velocity of the coffee ring flow within the evaporating sessile drop is calculated during the initial stage of this etching process when the spherical cap geometry assumption is valid. This is compared with the diffusion velocity of the disentangled polymer into the solvent. The two competing flows are used to elucidate why a hole could be formed initially. The complex wetting dynamics of the receding contact line is included to explain the via hole profile evolution in the later stage of the etching process and the existence of a penetration limit. These two stages are differentiated by the drop volume with respect to the volume of the via hole produced by the preceding drop. The competition between the coffee ring flow transferring polymer away from the central region and the polymer diffusion within the solvent drop is postulated to contribute to either via hole formation or a penetration limit, depending on which one of the two processes is dominant within the solvent evaporation time scale

The impact of substrate temperature on the size and aspect ratio of inkjet-dissolved via holes in thin poly(4-vinyl phenol) dielectric layers

The authors demonstrate the effect of substrate temperature on the relationship between the inkjet-etched via hole size and the number of drops of etchant dispensed. A mechanism for the different via hole size evolution versus the number of drops is proposed. An explanation for the interrelationship between the solvent evaporation rate and polymer re-deposition is presented. The aspect ratio of via holes produced is found to increase with the substrate temperature. Therefore, higher temperatures can be used to reduce the size and increase the aspect ratio of via holes fabricated by inkjet etching

Influence of etching solvent evaporation on the size of micro-via holes in PVP thin films

Via holes are a necessary component in traditional PCBs and IC interconnections. Such structures will also be required in organic electronics to achieve vertical communication between multiple layers. Inkjet printing has demonstrated its applicability in both hole creation and for other pattern generation requirements in various polymeric layers. However, the technique has not been systematically investigated. This paper is focused on a study of the effect of solvent evaporation rate on the size of inkjet-etched via holes for organic electronics, which is part of a more extensive investigation and evaluation of inkjet etching as a via hole fabrication technique. In this work, holes were etched in thin layers of poly(4-vinyl phenol) (PVP), which is a potential dielectric material for organic electronic structures. Ethanol, isobutanol and ethylene glycol were used as the etchants in order to study the effect of solvent boiling point and vapour pressure on the size evolution of via holes with the total number and the frequency of the solvent drops used to dissolve them. Isobutanol and ethylene glycol have higher boiling points than ethanol, leading to slower evaporation, which is believed to allow the dissolved polymer to flow backwards to the central area before complete solvent evaporation, resulting in hole refill. However it will be shown that applying temperatures higher than room temperature can accelerate solvent evaporation and eliminate the refill issue

The effect of droplet ejection frequency on inkjet-etched micro via holes

Inkjet etching has been identified as a potential route to formation of micro via holes in polymer dielectrics. Such vias could facilitate three-dimensional integration and sequential build-up fabrication in printed electronics. In the research reported in this paper, ethanol droplets were jetted onto a poly(4-vinyl phenol) (also known as PVP or PVPh) layer at different frequencies in order to observe the effect of droplet ejection frequency on the diameters of the via holes produced. The results demonstrate that via holes remain the same diameter at a low drop ejection frequency, while they enlarge at a relatively high frequency. A mechanism for this behaviour is proposed for which high speed photography provides evidence

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