Processing Ceramics With Lasers
Ceramics have been used in electronic applications for many years. However, with the recent commercial boom in electronic devices and the trend toward miniaturization, ceramic fabricators have sought more efficient and precise manufacturing methods and tools. The laser is one tool that has recently received much attention, in large part because advances in laser technology have improved the economics, efficiency and convenience of ceramic processing.
Laser processing of ceramics has a long history. However, laser use has primarily been limited to scribing applications involving fired alumina (A1203) substrates using fast- and slow-flow carbon dioxide (CO2) lasers. In the past, the size, reliability and operating cost of CO2 lasers prevented their widespread use in ceramics processing. But in recent years, technological advances have led to the development of entirely new sealed-CO2 and solid-state lasers that have overcome the drawbacks of early lasers, creating new applications and possibilities in ceramic processing.
Laser ApplicationsCeramic processing applications for lasers have increased since the development of several new industrial lasers. Lasers are now used to scribe, drill and profile, as well as for selective material removal and marking/serializing applications. They are used to process fired substrates of alumina, aluminum nitride (AlN) and beryllium oxide (BeO), as well as unfired (green) substrates (see Table 1).
Selecting the Right LaserA laser is a non-contact, zero-wear tool capable of precisely delivering enormous amounts of photon energy to a specific and highly localized area for drilling, cutting, scribing or welding. The type of interaction that takes place depends on the properties of the material to be processed, as well as the wavelength and energy of light emitted by the laser. Figure 1 shows the wavelength of light and range of output power emitted by various types of lasers.
Modern ceramic processing uses two types of lasers: CO2 lasers that emit in the far-infrared (FIR) region of the spectrum, and diode-pumped solid-state (DPSS) lasers that are available at output wavelengths ranging from the ultraviolet (UV) through the visible to the near-infrared (NIR).
CO2 LasersCO2 lasers emit in the FIR at a wavelength of 10.6 microns, which is easily absorbed by most ceramics. CO2 lasers also have high output powers, which allow high-speed processing and the processing of thick materials. Consequently, these lasers have been used by the ceramic industry for many years. However, in the past, only fast- and slow-flow CO2 lasers using flowing-gas technology (where the lasing gas mixture flows through the laser cavity) were employed in these applications.
Flowing-gas lasers exhibit excellent output laser beam properties with M2 (a parameter indicative of beam quality) values ranging from 1.1 to 1.3 (the ideal value being 1.0). However, despite the fact that flowing-gas laser beams are suitable for ceramic processing, several factors have prevented their widespread use and acceptance by the ceramic industry. First, flowing-gas lasers are relatively large devices (in excess of 8 ft in length) that occupy a considerable amount of shop floor space. Second, they are maintenance-intensive systems that require routine expert upkeep, including the replacement of optics and laser gases. Finally, the daily operating expenses of these lasers are high. For many years, these limitations made laser technology appear to be inconvenient, expensive and difficult to integrate.
Another advantage of the sealed, slab-discharge design for ceramic processing is its fast pulsing capability. Slab-discharge lasers emit high-frequency pulses (up to 100 kHz) with up to 1.5 kW of peak power. The combination of high frequency and high peak power results in faster material processing, minimal thermal degradation and a smaller heat-affected zone (HAZ).
Via (or hole) drilling is another ceramic processing application where sealed CO2 lasers are commonly used. Their high-frequency output allows sealed lasers to drill vias with speeds up to 200 holes/second in green ceramic and 1 to 4 holes/second in fired ceramic. Of course, drilling speed also depends on the thickness of material and the diameter of the vias. Due to the long emission wavelength of CO2 lasers, they are only used to drill vias down to 50 microns in diameter. For smaller features, short wavelength UV lasers are used with compatible materials.
In general, the average output power of sealed CO2 lasers is lower than that of flowing-gas lasers. However, in most ceramic processing applications only 20% of the usable power of flowing-gas lasers is used. This is because the use of the full power would increase both thermal damage and the HAZ. Consequently, 1-kilowatt flowing-gas lasers are usually replaced with 250 or 150 W sealed lasers. In some cases, 500 W sealed lasers are used, with the output of the laser split four ways to simultaneously process four ceramic machining stations.
One school of thought maintains that the high-quality output beam of flowing-gas lasers is worth the inconvenience and cost of operating them. This is a valid argument and applies to some applications where flowing-gas lasers currently are being used. However, new thinking suggests that the compact size; rugged, no-maintenance design; lower operating cost; and new HAZ-reducing features (high-frequency pulsing, fast rise-and fall-time pulses, and selective pulse shaping) of new sealed CO2 lasers compensate for their lower beam quality while producing higher quality holes in ceramics.
Diode-Pumped Solid-State (DPSS) LasersDPSS lasers used in ceramic processing are available in a variety of wavelengths, including 355 nm in the UV, 532 nm in the green portion of the visible spectrum, and 1064 nm in the near-infrared (NIR) region. The latest DPSS lasers contain one or more diode laser arrays that are used to continuously pump a rod of neodymium-doped vanadate (Nd:YVO4), and an acousto-optic Q-switch that is used to generate pulsed output. The fundamental output wavelength of Nd:YVO4 is in the NIR, at 1064 nm. A second and third harmonic crystal is then used to transform the beam from 1064 to 355 nm wavelength.
The latest DPSS lasers are typically air-cooled units and have lower facilities requirements than CO2 lasers. With their all-solid-state design, they are also highly compact and rugged devices. Originally, solid-state lasers were flashlamp-pumped devices with low reliability, due to their aging and the thermal degradation of flashlamps, which had to be replaced every few hundred hours at great expense. However, leading manufacturers of today’s DPSS lasers offer a minimum guaranteed time-to-service of 5,000 to 10,000 hours.
DPSS lasers are used for a variety of applications in the ceramic industry. Marking and serializing are most commonly performed with NIR DPSS lasers at 1064 nm, due to their high processing speeds and minimal thermal damage. However, marking and serializing are sometimes performed with green (532 nm) and UV-DPSS (355 nm) lasers as well. DPSS marking speeds on a variety of materials range between 100 and 500 mm/second at 1064 nm wavelength.
UV-DPSS lasers are used in ceramic processing applications where CO2 and other long wavelength lasers are unsuitable. These applications usually involve small feature sizes or require high-quality processing where no thermal degradation is tolerated and where speed of manufacturing is not the primary concern. The short wavelength of UV-DPSS lasers (355 nm) compared with CO2 lasers (10,600 nm) allows them to focus down to smaller spots. Therefore, UV-DPSS lasers can more reliably drill smaller features (25 mm), such as vias, than CO2 lasers (70 mm).
Compared to CO2 lasers, UV-DPSS lasers have relatively low output powers. Consequently, they are only used for processing thinner, uncured green ceramics. However, UV-DPSS lasers with a 50% increase in output power have recently been developed. These units have average output powers in excess of 4.5 W at 355 nm. In the future, units with even higher output power are expected, making these devices even more cost-effective.
Bringing Lasers MainstreamNew CO2 and DPSS laser technology has provided the potential to improve the economics and convenience of ceramics processing. Modern, no-maintenance sealed CO2 lasers featuring slab-discharge technology offer high-frequency pulsing and pulse shaping that improve feature quality and reduce HAZ. DPSS lasers are also now available in UV, visible and wavelengths with higher output powers. The convenience and cost effectiveness of these new lasers are making laser processing of ceramics a mainstream technique.
For More InformationFor more information about CO2 and DPSS lasers, contact Sri Venkat at Coherent Photonics Group, Laser Division, 5100 Patrick Henry Drive, Santa Clara, CA 95054; (408) 764-4446; fax (408) 764-4800; e-mail firstname.lastname@example.org; or visit http://www.coherentinc.com.
*The Coherent® Diamond™ laser.