Expanding Markets on a Miniature Scale

June 1, 2004
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High-precision laser micro-machining is enabling the cost-effective creation of extremely small features in a wide range of ceramic components

Components made from alumina (Al2O3), silicon nitride (Si3N4), aluminum nitride (AlN) and other ceramic materials are used in a wide range of electronic, automotive, aerospace, medical and other high-tech applications. As these application areas continue to grow, manufacturers must find new and better solutions for machining these materials so that they can produce parts more cost effectively and on an increasingly smaller scale. While lasers have been used to drill, cut and scribe ceramics for many years, the latest generation of laser systems offers new micro-machining possibilities in ceramics.

The Evolution of Lasers

Traditional mechanical machining/drilling of fired Al2O3 and other ceramic materials is difficult and expensive due to the hardness and brittleness of these materials. Sawing or scribing can be accomplished using diamond blades; however, this method results in large kerf widths, blade wear and increased production costs, and it is impractical for cutting complex profiles. Mechanical machining methods become even more unattractive as the requirement for smaller and more accurate features increases. Electrical discharge machining (EDM) can be used on certain conductive materials but does not have a broad application range.

Lasers, on the other hand, are not limited by the type or hardness of the material and can be used for complex profiling. The first types of lasers used were flowing gas carbon dioxide (CO2) and latterly sealed CO2 lasers. These lasers are able to drill, cut and scribe ceramics at high speeds (up to 15 m/min scribe rates). The laser pulse melts the material, which is then removed by a mixture of melt shearing and pressurized ejection (with the aid of a high-pressure coaxial gas jet). However, the minimum kerf width or hole size is limited to about 50 µm due to the long wavelength (10.6 µm) of these lasers. To get below this limit with high precision, other laser sources must be used. Nd:YAG lasers with a 1.064 µm wavelength offer higher precision than the CO2 laser, but the material removal process still occurs through melting. As a result, glassification of the hole or cut wall, and sometimes resolidified droplets, can occur on the machined part, making the process unsuitable for micro-machining processes.

More recently, industrial lasers with shorter wavelengths (visible and UV) and higher peak power pulses have become available. These lasers process the material through an ablation mechanism in which the material is largely vaporized rather than melted. Visible and UV laser micro-machining has become a standard method of manufacturing precision parts in a number of industries; for example, lasers have been used for some time in the electronics industry to manufacture a host of components for the circuits of mobile phones or laptops. Advances in lasers and processing techniques have widened the range of parts that can be manufactured and have led to vast improvements in accuracy.

Visible and UV Laser Machining

Today's lasers can be used for a variety of processes to produce feature sizes in the 1- to 500-mm range with submicron accuracy. Since lasers range in power from sub-mW to many kW, it is important to understand the process and to choose the correct laser for the application.

For precise material removal, pulsed lasers are best, and average powers in the 1 to 100 W range are generally used. The CO2 (10.6 µm) and fundamental Nd:YAG lasers (1064 nm) can be used to create larger, less precise features. However, when highly precise features with smaller dimensions are required, copper vapor lasers (511 and 578 nm), frequency-doubled Nd:YAG or vanadate lasers at 532 nm, and tripled YAG or vanadate at 355 nm should be used. All of these visible or UV lasers have pulse widths of a few to a few tens of nanoseconds (resulting in very high peak power pulses), with repetition rates of 0-100 kHz.

The simplest laser processing technique is percussion drilling. In this technique, the laser light is focused to a small round spot. The size of the feature machined is determined by the size and shape of the focal spot, while the depth is determined by the pulse duration, energy and number of pulses used. The percussion technique allows the drilling of many holes very quickly-typically hundreds of holes per second. For some applications, however, percussion drilling does not provide the accuracy or shape of feature required. In recent years, with the improvements in diode-pumped solid-state lasers, laser beams have become very circular, but they still are not 100% round. To achieve greater accuracy and shape flexibility in applications such as probe head dies, which require high-tolerance circular holes, systems can be used that more precisely align the workpiece and laser beam. For example, very high precision circular holes (±0.2 µm) can be drilled using an optical trepanning technique, in which optical elements are used to spin the beam. This allows the beam to spiral or traverse the circumference of a circle while remaining focused on the workpiece. Larger parts and more complex shapes can be made using multi-axis machining systems, including galvanometer scanning heads, which can effectively mill away the material.

Figure 1. A 500-µm-thick Si3N4 component with 70-µm-diameter holes on a 150-µm pitch, drilled with a copper laser.

High-Precision Components

Laser micro-machining is helping manufacturers of ceramic components expand their product offerings and market share in areas such as probe heads, diode laser mounts, fiber interconnects and sapphire scribing.

Probe Heads. In the electronics industry, laser micro-machining is increasingly being employed to create components used in vertical probe cards for in-situ wafer testing. The probe cards, which consist of hundreds or thousands of finely spaced needles, are touched to the wafers for testing. Within these probe cards, several dies manufactured from Al2O3 or Si3N4 and composed of fine-pitched arrays of accurately positioned holes (typically within ±2 µm) are used to mount and guide the needles. A close-up picture of a probe head die is shown in Figure 1. Although these holes are circular, they can also be rectangular, depending on the application.

Figure 2. This AIN diode bar has been cut, drilled and milled with a visible, short-pulse laser.
Diode Laser Mounts. The ability to precisely machine components made from materials such as AlN allows manufacturers to produce small components for diode laser mounts. The starting point for the AlN component shown in Figure 2 was a small bar of material; the figure clearly demonstrates the laser's ability to cut, drill and mill precise features in this material.

Figure 3. A laser with trepan techniques was used to create this ceramic four-fiber interconnect.
Fiber Interconnects. In addition to drilling blind holes and through holes, modern lasers can also control the taper through the bore of particular holes when used with trepan techniques. This technique was used to create the four-fiber interconnect shown in Figure 3. In this application, the entrance diameter (top) had to be larger than the exit (bottom) and tapered down in a controlled way. The hole diameter at the bottom of the connector is 130 microns, while the diameter at the top is 200 microns. (The fifth hole in this component is necessary to release air from the connector when the fibers are pushed in.)

Figure 4. This small sapphire component was machined using a diode-pumped solid-state laser running at 266 nm.
Sapphire Scribing. In the last few years, engineers have been turning to lasers for cutting and scribing sapphire-particularly GaN components supported on sapphire wafers for the blue light emitting diode (LED) market. Figure 4 shows a small component machined from sapphire using a diode-pumped solid-state laser running at 266 nm.

Enabling Future Advances

As the drive to create lighter and less expensive ceramic components increases, so will the need for more precise processing techniques. Laser micro-machining can be used in a wide variety of applications, spanning industries from electronics and aerospace to automotive and medicine. The use of lasers in precision manufacturing is growing rapidly, and will undoubtedly open the door to further miniaturization of many devices.

For more information about laser micro-machining, contact Oxford Lasers, Unit 8, Moorbrook Park, Didcot, OX11 7HP UK; (44) 1235-814433; e-mail alan.ferguson@oxfordlasers.com ; or visit http://www.oxfordlasers.com .

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