Laser techniques in glass joining

Opportunities for biomedical and aerospace industries

 

NINA GARLO-MELKAS

 

Fusion welding is one of the most promising techniques in glass joining today. Both ecological and cost-efficient, it can be accomplished without any intermediate layer and, with the laser production steps, can be done in a non-contact manner. High precision without heat affect makes the ultrashort pulsed laser perfect in the performance of such a process, as it produces minimal damage to the surrounding area. The laser also creates new possibilities for a wealth of industries, including the fast-growing biomedical sector and the high-demand aerospace industry.

The melting and fusing together of two pieces of material to make one is a key process utilized in much of today's industrial world—it is estimated, for instance, that the safety of a single manufactured car depends in part on the reliability of more than 3000 welds! The welding of metals is a somewhat straightforward process due to that element's ability to be deformed without breaking. Until recently, the joining of pieces of glass has been a bigger challenge for scientists and industrial manufacturers alike because of the specific characteristics of the material, which, while being optically transparent, is highly sensitive to cracking and breakage due to changes in temperature, among others.

Isamu Miyamoto, Emeritus Professor at the University of Osaka in Japan, a pioneer in the exploration of the physical details behind today's laser-based glass welding processes, together with Finnish scientists, has examined how the utilization of ultrafast lasers in the process of glass welding results in a higher efficiency of manufacturing processes and increased durability of its final products (FIGURE 1). "Today's laser microwelding technology produces an extremely small heat-affected zone (HAZ); thus, there is no risk of destroying sensitive or even organic components. This expands the potential for packaging fragile components, including under or inside glass, which had been a challenge for manufacturers in the past years," says Jarno Kangastupa, one of Finland's leading experts in laser technology, who worked in scientific collaboration with Miyamoto.

A Primoceler lab engineer examines a welded part
FIGURE 1. A Primoceler lab engineer examines a welded part. (Photos courtesy of Mikko Henrik Huotari)

Opportunities for industry

According to Kangastupa, the utilization of ultrashort laser pulses provides many benefits to glass-to-glass welding because it means that no additional layers on substrates or adhesives are required in the process, unlike in the commonly utilized eutetic or adhesive bonding method. It also requires far less energy than other traditionally used glass-to-glass welding techniques such as anodic bonding—also known as field-assisted bonding—or electrostatic sealing due to its utilization of electric fields in the welding process.

Kangastupa notes that packaging is a key element of microelectromechanical systems (MEMS) design, where the environment inside the package is vital to the efficacy of the MEMS device application. This is particularly true for the MEMS sensor segment that already produces glass-encapsulated electronic chips used for sensing rotation, acceleration, and pressure in various safety-critical applications in the automotive, train, and other transportation industries. It is also the case for many applications developed in the fast-growing biomedical segment.

A key benefit of today's laser-based welding solutions (FIGURE 2) is that it enables safer encapsulation of sensitive materials from the effects of high temperatures or chemical substances, which are produced in other existing welding techniques.

Primoceler lab engineers work on the laser welding unit
FIGURE 2. Primoceler lab engineers work on the laser welding unit.

"Today's laser-based glass-to-glass welding results in separate pieces of glass joined together, becoming in a way one single piece of material," according to Kangastupa. "This leads to good mechanical properties of the welding seam—a property that is highly important in the aero and space industry. In space conditions, welded objects, including encapsuled microchips devices such as CMOS (complementary metal oxide semiconductor) image sensors, must remain highly reliable and airtight, even in the toughest of conditions, such as in extreme weather conditions, and having to face a significantly increased threat of radiation damage."

He states that existing technology makes it possible to bond pieces of glass together hermetically without the need for high temperatures or adhesives—even the sealed component or material remains unharmed by the welding process itself.

"Advanced laser processes involve a highly focused laser beam that lets various components sealed inside the encapsulation to remain under room temperature," Kangastupa continues. "This presents manufacturers with an array of new possibilities for the production of electronic, engineering, medical, and scientific research devices such as implantable chips and sensors."

"Unlike the joining of glass with adhesive techniques, the seam produced via glass-on-glass laser welding is practically eternal because it doesn't include any adhesives that could gradually turn brittle following the slow evaporation of its chemicals," he adds.

Building better medical diagnostics

Jorma Vihinen, research manager at Tampere University of Technology, agrees that in the future, laser-based microwelding methods will spread strongly in the biomedical industry. New applications are likely to arise rapidly, for instance, in the field of human medicine, as the knowledge of new laser-based, glass-to-glass sealing methods increases among industrial players.

Why is the joining of glass so important for the biomedical industry? Glass as a material is highly suitable for biomedical purposes due to its useful properties. First, glass is "neutral" in a sense that it is biocompatibile with body fluids when implanted inside the human body. This means that it doesn't cause immunological rejection to the implant that would result in the need to remove the object from the body. Secondly, the lifespan of glass is essentially infinite: if it ends up in a landfill, it can last for several hundreds of thousands of years. Glass doesn't wear, tear, or gradually degrade like the many adhesives or additional substrates used in other available welding processes do. Glass is also transparent to radio frequency (RF) wavelengths, which allows the transfer of data or energy possible through the glass-encapsulated device, unlike titanium-packaged applications.

Vihinen notes that as technology continues to advance, laser-based glass-to-glass welding solutions could quickly and cost-effectively assist in the production of new in vitro diagnostic devices that help detect various diseases, physiological conditions, and characteristics of the body's organisms' conditions and diseases, possibly replacing time-consuming and expensive laboratory tests. For instance, the incidence of a heart attack may be able to be detected, in the future, instantaneously from a single blood sample.

Moreover, research shows that brain damage and memory disorders could someday be treated with new high-tech brain implants governed by human emotion and could even guide our thinking. Studies suggest that brain implants could also enhance human abilities by quickening reaction times, promoting a person's learning ability, and even improving memory.

Adding flexibility to manufacturing

Glass-on-glass welding offers opportunities in the electronics industry, where handheld devices are continuing to shrink in size. Miniaturization will continue to be a trend as long as consumers continue to be interested in portability. A handheld mobile phone today is already totally different now from what it was when first introduced by Motorola in the 1970s—when it weighed a whopping 2.2 lbs!

Kangastupa says, "When the innovations in new glass materials and laser processes are combined, we can foresee continued part reduction and miniaturization in mobile devices. It is possible to integrate all the features needed for display devices on a very thin glass and, in a final step, laser-weld the second glass on top to protect the display. The second glass with a hermetic weld line can have more functions than protection against environment. It can include thin-film layers needed for touch-panel and antireflection properties, or for some new features which are today available only in the laboratories."

He adds that laser-based glass-on-glass welding could also deliver cost savings to manufacturers, as it enables a more flexible production process. Manufacturers can continue with trial runs to assess how a particular product works, or they could manufacture small production series first until they are confident about the production processes' final outcome.

According to Kangastupa, "If needed, even one encapsulation of, for instance, a microchip can be done at a time to ensure that everything goes right before full-scale production begins."


NINA GARLO-MELKAS is with Primoceler, Inc., Tampere, Finland.

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