Acerca de las funcionalidades obtenidas

Integration of all process steps results in manufacturing of personalized glass products. SACE machining of different features in glass like holes, channels, microfluidic mixers, though-glass-vias, contour cutting and engraving of hardened smartphone cover glasses is achieved with high machining accuracy (typically with geometrical errors below 10 µm compared to the design of the customer). Results of this variety of glass applications, machined by SACE technology on demand, are presented in Figure 4.4.

The typical machining voltage applied across tool- and counter electrode was pulsed voltage between a high voltage (32 V) and a low voltage 17.5 V) with period = 2.6 ms, and duty cycle =

96.15 %. During machining, the tool was rotated at 500 rpm and a feedrate of 200 µm/s was used.

As electrolyte, 20 wt.% KOH was used in most cases.

Figure 4.4. Examples of personalized micro-machining structures in glass by SACE process.

4.4. Conclusions

The results of this study show the feasibility of an approach using SACE to achieve personalized parts of hard-to-machine materials such as glass with a low-cost machining setup adding to novel manufacturing approaches fitting Industry 4.0. Key is the use of low-cost rapid prototyping technology and an in-situ fabrication method for the needed tooling, reducing costs and lead times compared to conventional SACE machining approaches. Traditional manufacturing techniques are typically optimized for one or few different operations, while hybrid technologies as SACE have a great potential to support manufacturing in the Industry 4.0 approach as they are flexible and can reduce the overhead associated with each specific manufacturing technique to allow mass personalization at reasonable cost. The presented manufacturing methodology is a

Micro structures Microfluidic chip Micro mixer Hot embossing template

Watch component Consumer electronics Through-Glass-Vias Smartphone cover glass

Biomedical device Micro hinge

good example of process adaptation to satisfy changing requirements such as geometries or volume to achieve mass personalized goods.

Besides these assets, the proposed method has some other significant advantages compared to traditional glass machining technologies, such as wet etching or abrasive jet machining:

• The product design files can be used straight forward in the manufacturing process without the need of additional steps as e.g. layered masks in wet etching techniques;

• Almost no restrictions for the desired geometry to be machined e.g. no taper restrictions as in abrasive jet machining;

• 2.5 D structures in glass can easily be machined.

This approach can be used for fabrication of high precision applications made of glass such as smart phones, advanced medical devices like Lab-on-Chip, green energy devices and fiber optic telecommunications.

Chapter 5

Bonding, being the last and most critical step in most glass micro-device manufacturing (e.g.

microfluidic chips) can be a main factor of failure of glass chip manufacturing. Especially, direct glass-to-glass bonding after micro-structuring features in glass remains a challenging task at batch size 1. Most often necessary intermediate steps between machining and bonding jeopardize one of the key requirements for batch size 1 manufacturing – reduction of processing steps in the manufacturing workflow. Large batch processes such as wet etching do not require extensive post-processing steps before proceeding to bonding, however these batch processes are not appropriate for batch size 1 production by its parallel nature and complexity (e.g. requiring use of cleanroom, masking and extensive alignment operations). On the other hand, flexible technologies (e.g. Laser, mechanical machining) need extensive post-processing steps before bonding can be performed.

Hybrid technologies as SACE are interesting to explore on its performance as machining step before glass-to-glass bonding.

This chapter details the study on deploying SACE technology as machining step in the fabrication of packaged glass devices (glass-to-glass bonded). It is shown by fabrication of a simple microfluidic Y-mixer that using SACE machining eliminates a post-processing step after machining of the desired structure on a glass substrate for subsequent glass-to-glass bonding. This method provides a solution to reduce one of the manufacturing cost drivers – reduction of manufacturing process steps. Quality of the achieved glass-to-glass bonding is assessed both qualitatively and quantitatively by the razorblade insertion test, acoustic imaging (Sonoscan®), electron microscopy (SEM) and leakage testing by microfluidic mixing at high pressure.

Rapid prototyping of packaged glass devices:

eliminating a process step in the manufacturing workflow from micromachining to die singularizing

Lucas A. Hof and Rolf Wüthrich

Department of Mechanical & Industrial Engineering, Concordia University, 1455 de Maisonneuve Blvd.

West, Montreal, QC H3G 1M8, Canada

This article has been published in Manufacturing Letters, 17, August 2018, Pages 9-13

Abstract

Direct glass-to-glass bonding after micro-patterning in glass is often challenging, especially in the frame of rapid prototyping, as several special cleaning or other post-processing steps are needed before bonding is possible. In this study, we demonstrate that glass-to-glass bonding is possible directly after Spark Assisted Chemical Engraving (SACE) micromachining without any special post-treatments. This approach enables flexible prototyping of glass devices at relatively low cost, which is illustrated by fabrication of functional microfluidic devices. The machined and bonded glass device is evaluated both qualitatively and quantitatively on performance and shows good results.

Keywords: glass-to-glass bonding; microfabrication; rapid prototyping; Spark Assisted Chemical Engraving (SACE); microfluidics

5.1. Introduction

Lab-on-a-chip (LOC) and other microfluidic devices are widely used in various research fields including life science and diagnostics [72, 92, 101, 110]. While there is a clear trend to use

low-cost materials like PDMS for disposable devices, there is still a wide range of applications which demands glass as substrate material [75]. This is mainly because of its unique properties, like optical transparency, chemically inertness, well known surface chemistry, biocompatibility, thermal properties and mechanical strength.

There is as well a need for rapid prototyping of such devices. Key challenges for fabrication of glass devices, particularly in low batch sizes, are machining, due to the hardness and brittleness of glass [83, 138] and subsequent bonding [259, 260] to seal the device.

Bonding is the last and most critical step in microfluidic chip manufacturing, which can be one of the main factors resulting in failure of glass chip manufacturing [101, 259, 261]. Common methods for glass bonding include anodic [262], thermal fusion [262], and adhesion bonding [263], where each category represents a wide variety of techniques for specific applications. Successful glass bonding requires properly polished and clean surfaces (root mean square surface roughness

< 0.6 nm [264]) without any irregularities [261, 265-267]. Intermediate steps are usually necessary after machining, jeopardizing the desired low-cost rapid prototyping approach.

Parallel batch processes, such as wet etching [94], for machining relatively low aspect-ratio structures in glass are well established and do not present excessive difficulties for glass-to-glass bonding. However, when using flexible technologies for low-cost rapid prototyping of glass devices the bonding step becomes a major challenge [268]. Thermal processes like LASER are fast and flexible but form bulges near the machining zone in glass, which leads to bonding difficulties, making post process steps necessary [138, 267, 269, 270]. Mechanical methods, such as diamond tool drilling or powder blasting, have to deal with the issue that small glass debris or abrasive particles can easily stick to the glass substrate leading to bonding defects [118, 271].

Hybrid technologies like spark assisted chemical engraving (SACE) [81, 186] are interesting as they attempt to combine the advantages of each process to satisfy most requirements for low-cost rapid prototyping of micro-structures in glass, which have to be subsequently bonded for device fabrication. In SACE technology, a voltage is applied between tool and counter electrode dipped in an alkaline solution. At high voltages (around 30V), the bubbles evolving around the tool electrode coalesce into a gas film. Discharges occur from the tool to the electrolyte through this gas film [186]. Glass machining happens by thermally promoted etching (breaking of Si-O-Si bonds) [221].

Bonding a glass cover on the top of a device micro-machined by SACE was not yet investigated and it is an open question if major intermediate steps are necessary before being able to successfully proceed to bonding.

In this study, the bonding of two glass wafers directly after micromachining by a low-cost prototyping micromachining method (SACE technology) is investigated and it is shown that no major intermediate steps are necessary for successful subsequent glass-to-glass bonding.